Use a default SLUG_ID value
Use a default SLUG_ID value

otherwise the build fails using the Heroku beta
[builds api](https://gist.github.com/friism/c5df3e3e0091bbc6b9f2)

Brogency/heroku-buildpack-python-gulp,kvarga/heroku-buildpack-python,pkumar-appd/python-buildpack,gitricko/apt-python-buildpack,DylanLukes/heroku-buildpack-python,marcospy/heroku-buildpack-scipy,HousekeepLtd/heroku-buildpack-python,ephemerallabs/heroku-buildpack-python,jvenezia/heroku-buildpack-python,benjaminfox/heroku-buildpack-python-geoip,heroku/heroku-buildpack-python,alexflanagan/operand-buildpack-python,predata/heroku-buildpack-scipy,mobify/heroku-buildpack-python-swig,edenspiekermann-amsterdam/heroku-buildpack-python,cloudfoundry/python-buildpack,joshowen/heroku-buildpack-python,Nazanin84/python-buildpack,joshisa/python-buildpack,gitricko/apt-python-buildpack,arnaudlimbourg/heroku-buildpack-python,tommyvn/heroku-buildpack-python,vlidholt/heroku-buildpack-scipy,pmclanahan/heroku-buildpack-python-peep,maxcountryman/heroku-buildpack-python,manufacturedba/heroku-buildpack-python,igroff/heroku-buildpack-python,mythmon/heroku-buildpack-python-nodejs,Tiger66639/buildbot-master-buildpack,ywarezk/heroku-buildpack-geodjango,martingascon/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,Scalingo/python-buildpack,nikita-pivovarov/heroku-buildpack-python-ldap,nemesiscodex/heroku-buildpack-python,RobertSundling/heroku-buildpack-python-nginx,joshfriend/heroku-buildpack-python,blazekadam/open2,cclauss/heroku-buildpack-python,swanson/heroku-buildpack-python-ortools,driftyco/heroku-buildpack-sphinx,samuelhavron/heroku-buildpack-python,YoApp/heroku-buildpack-python-twemproxy,agriffis/heroku-buildpack-python,papaeye/heroku-buildpack-python-bower,ruscoder/heroku-buildpack-python,maciejgryka/heroku-buildpack-spcedge,dvreed/heroku-buildpack-python-ldap,edmorley/heroku-buildpack-python,andrespires/python-buildpack,bobsilverberg/heroku-buildpack-python-peep,brightforme/heroku-buildpack-python,gaumire/heroku-buildpack-python,guidowb/buildpack-python,samuelhavron/heroku-buildpack-python,ojacobson/heroku-buildpack-python,monological/heroku-buildpack-python-opencv,grivescorbett/heroku-buildpack-python,slorg1/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,smalleats/heroku-buildpack-python,buildbot/buildbot-master-buildpack,guidowb/buildpack-python,scrapinghub/scrapinghub-buildpack-scrapy,andrespires/python-buildpack,river-jade/heroku-buildpack-scipy,projetoeureka/heroku-buildpack-scipy,18F/python-buildpack,TipWander/heroku-buildpack-geodjango,liyanchang/cloudfoundary-buildbot-buildpack,skilljar/heroku-buildpack-python,brianthomas/heroku-buildpack-textmining,nwkotto/geodjango-buildpack-with-geos,liyanchang/buildbot-master-buildpack,loanzen/heroku-buildpack-python,liyanchang/cloudfoundary-buildbot-buildpack,cloudfoundry/python-buildpack,nadirizr/heroku-buildpack-python-gulp,rollokb/heroku-buildpack-python,samuelhavron/heroku-buildpack-python,Stibbons/heroku-buildpack-python-installer,marcospy/hackathon-globo-app,sjtuadamyang/adamyang-buildpack-python-libjpeg,thenovices/heroku-buildpack-scipy,ryanhiebert/heroku-buildpack-python,Mause/heroku-buildpack-python,plangrid/heroku-buildpack-python,jimlindeman/python-buildpack-collectd,zakdoek/heroku-buildpack-django,18F/python-buildpack,icoxfog417/heroku-buildpack-python,jimlindeman/python-buildpack-collectd,kowito/python-buildpack,miraculixx/heroku-buildpack-python,resmio/heroku-buildpack-geodjango,stalnacke/heroku-buildpack-python,guidowb/buildpack-python,pebble/cloudpebble-buildpack,pwdel1/heroku-buildpack-scipy,AltSchool/heroku-buildpack-python,zakdoek/heroku-buildpack-django,dulaccc/heroku-buildpack-geodjango,doda/heroku-buildpack-python,doda/heroku-buildpack-python,lukaszkorecki/heroku-buildpack-python,Apogaea/heroku-buildpack-python,benjaminfox/heroku-buildpack-python-geoip,rbtying/heroku-buildpack-python,ad-m/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,wbrefvem/heroku-buildpack-python,ianyh/heroku-buildpack-python-bower,integricho/heroku-buildpack-python-ffmpeg2.1-with-codecs,samuelhavron/heroku-buildpack-python,heroku/heroku-buildpack-python,wbrefvem/heroku-buildpack-python,ad-m/heroku-buildpack-python,hsr-ba-fs15-dat/heroku-buildpack,jamarparris/heroku-buildpack-python-swig,gitricko/apt-python-buildpack,amanjain/heroku-buildpack-python-with-django-bower,PerceptumNL/heroku-buildpack-python,marianzange/heroku-buildpack-python,Nazanin84/python-buildpack,samuelhavron/heroku-buildpack-python,kowito/python-buildpack,adamroth/heroku-buildpack-python-pdftk,18F/python-buildpack,oberlin/heroku-buildpack-python,junyoed/heroku-buildpack-python,xica/heroku-buildpack-scipy,joshisa/python-buildpack,Nazanin84/python-buildpack,marcospy/hackathon-globo-app,samuelhavron/heroku-buildpack-python,cortexintel/heroku-buildpack-scipy,pkumar-appd/python-buildpack,AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,beni55/heroku-buildpack-python,liyanchang/cloudfoundary-buildbot-buildpack,joshisa/python-buildpack,ekkleesia2/heroku-buildpack-python,andela-brotich/wger-buildpack-python,WonderThink/heroku-buildpack-python-ffmpeg-geoip,markine/heroku-buildpack-python,swils/heroku-buildpack-python,cloudfoundry/python-buildpack,dubsmash/heroku-buildpack-python,soapril/heroku-buildpack-python,alexflanagan/operand-buildpack-python,marcospy/hackathon-globo-app,nicktimko/heroku-buildpack-scipy,cdht/heroku-buildpack-python,rapilabs/heroku-buildpack-python,kowito/python-buildpack,jimlindeman/python-buildpack-collectd,samuelhavron/heroku-buildpack-python,pkumar-appd/python-buildpack,andrespires/python-buildpack,HanSooloo/heroku-buildpack-python