Update .travis.yml
Update .travis.yml

* Fix an issue in .travis.yml where CUSTOM_JDK environment variable is not prefixed with a dollar sign.
* Fix an issue in .travis.yml where OSX doesn't have any JDK specified.
ahartley39/DataflowJavaSDK,jasonkuster/beam,apache/beam,Test-Betta-Inc/musical-umbrella,yk5/beam,iemejia/incubator-beam,elibixby/DataflowJavaSDK,wtanaka/beam,nagachika/DataflowJavaSDK,charlesccychen/incubator-beam,charlesccychen/incubator-beam,chamikaramj/beam,vikaspalkar/DataflowJavaSDK,chamikaramj/beam,RyanSkraba/beam,chamikaramj/MyDataflowJavaSDK,robertwb/incubator-beam,wangyum/beam,RyanSkraba/beam,yk5/beam,rangadi/incubator-beam,eljefe6a/incubator-beam,sammcveety/incubator-beam,dhalperi/incubator-beam,jbonofre/beam,tgroh/beam,chamikaramj/beam,ravwojdyla/incubator-beam,peihe/DataflowJavaSDK,sammcveety/DataflowJavaSDK,chamikaramj/beam,markflyhigh/incubator-beam,elibixby/DataflowJavaSDK,tgroh/beam,nevillelyh/DataflowJavaSDK,springml/DataflowJavaSDK,tgroh/incubator-beam,robertwb/incubator-beam,dhalperi/beam,tyagihas/DataflowJavaSDK,amitsela/beam,twcctz500000/DataflowJavaSDK,markflyhigh/incubator-beam,PieterDM/DataflowJavaSDK,staslev/incubator-beam,twcctz500000/DataflowJavaSDK,haocafes/DataflowJavaSDK,apache/beam,xsm110/Apache-Beam,chamikaramj/beam,lukecwik/incubator-beam,rangadi/beam,charlesccychen/beam,jbonofre/beam,dhananjaypatkar/DataflowJavaSDK,dhalperi/beam,haocafes/DataflowJavaSDK,robertwb/incubator-beam,prabeesh/DataflowJavaSDK,apache/beam,apache/beam,markflyhigh/incubator-beam,peihe/incubator-beam,joshualitt/incubator-beam,chamikaramj/incubator-beam,charlesccychen/incubator-beam,haonature888/DataflowJavaSDK,dhananjaypatkar/DataflowJavaSDK,lukecwik/incubator-beam,josauder/AOP_incubator_beam,sureshjosh/DataflowJavaSDK,amitsela/incubator-beam,apache/beam,joshualitt/incubator-beam,yk5/beam,manuzhang/beam,tweise/incubator-beam,rangadi/beam,charlesccychen/beam,sammcveety/incubator-beam,markflyhigh/incubator-beam,manuzhang/beam,tweise/beam,sammcveety/incubator-beam,markflyhigh/incubator-beam,apache/beam,peihe/DataflowJavaSDK,tomwhite/DataflowJavaSDK,vikkyrk/incubator-beam,dhalperi/incubator-beam,lukecwik/incubator-beam,joshualitt/DataflowJavaSDK,vemmaverve/DataflowJavaSDK,RyanSkraba/beam,charlesccychen/beam,jbonofre/incubator-beam,charlesccychen/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,staslev/beam,tgroh/beam,markflyhigh/incubator-beam,RyanSkraba/beam,mxm/incubator-beam,ravwojdyla/incubator-beam,apache/beam,sureshjosh/DataflowJavaSDK,manuzhang/incubator-beam,vikaspalkar/DataflowJavaSDK,apache/beam,ahartley39/DataflowJavaSDK,mxm/incubator-beam,tgroh/beam,Test-Betta-Inc/musical-umbrella,wtanaka/beam,staslev/beam,robertwb/incubator-beam,chamikaramj/beam,amitsela/beam,springml/DataflowJavaSDK,robertwb/incubator-beam,charlesccychen/beam,markflyhigh/incubator-beam,xsm110/Apache-Beam,apache/beam,vikkyrk/incubator-beam,chamikaramj/beam,rangadi/beam,chamikaramj/beam,vikkyrk/incubator-beam,Priyalc/Gcloud,wangyum/beam,staslev/beam,amarouni/incubator-beam,amitsela/incubator-beam,josauder/AOP_incubator_beam,chamikaramj/incubator-beam,jasonkuster/incubator-beam,apache/beam,jbonofre/beam,amitsela/beam,amarouni/incubator-beam,charlesccychen/beam,haonature888/DataflowJavaSDK,wangyum/beam,PieterDM/DataflowJavaSDK,chamikaramj/beam,tweise/beam,dhalperi/beam,tweise/incubator-beam,haonaturel/DataflowJavaSDK,charlesccychen/beam,robertwb/incubator-beam,rangadi/incubator-beam,haonature/DataflowJavaSDK,tyagihas/DataflowJavaSDK,robertwb/incubator-beam,joshualitt/DataflowJavaSDK,wtanaka/beam,chamikaramj/beam,apache/beam,iemejia/incubator-beam,manuzhang/incubator-beam,wangyum/beam,peihe/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,jasonkuster/beam,rangadi/beam,rangadi/beam,Priyalc/Gcloud,manuzhang/beam,smarthi/DataflowJavaSDK,robertwb/incubator-beam,RyanSkraba/beam,smarthi/DataflowJavaSDK,staslev/incubator-beam,chamikaramj/MyDataflowJavaSDK,eljefe6a/incubator-beam,rangadi/beam,ravwojdyla/incubator-beam,rangadi/beam,haonaturel/DataflowJavaSDK,peihe/incubator-beam,jasonkuster/beam,nagachika/DataflowJavaSDK,RyanSkraba/beam,RyanSkraba/beam,sammcveety/DataflowJavaSDK,tgroh/incubator-beam,jbonofre/incubator-beam,jbonofre/beam,jasonkuster/incubator-beam,eljefe6a/incubator-beam,robertwb/incubator-beam,prabeesh/DataflowJavaSDK,vemmaverve/DataflowJavaSDK,xsm110/Apache-Beam,tomwhite/DataflowJavaSDK,robertwb/incubator-beam,lukecwik/incubator-beam,rangadi/incubator-beam,haonature/DataflowJavaSDK,lukecwik/incubator-beam