Update A_up_and_running.md
Update A_up_and_running.md

* Remove relative paths
* Install hex before moving forward
* Mention rebar installation may be prompted
Gazler/phoenix,leikind/phoenix_guides,bratsche/phoenix_guides,korobkov/phoenix,0x00evil/phoenix,larrylv/phoenix_guides,stevedomin/phoenix,thetamind/phoenix_guides,gjaldon/phoenix,stevedomin/phoenix_guides,tokafish/phoenix_guides,evax/phoenix,mobileoverlord/phoenix,0x00evil/phoenix,Gazler/phoenix,mydearxym/phoenix_guides,eksperimental/phoenix,wsmoak/phoenix_guides,dmarkow/phoenix,keathley/phoenix,optikfluffel/phoenix,pap/phoenix-guides,sakishum/phoenix_guides,mschae/phoenix,0x00evil/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,1ternal/phoenix_guides,evax/phoenix,rafbgarcia/phoenix,dmarkow/phoenix,korobkov/phoenix,nshafer/phoenix,rafbgarcia/phoenix,jeffkreeftmeijer/phoenix_guides,tuvistavie/phoenix_guides,iguberman/phoenix_guides,michalmuskala/phoenix,pap/phoenix,CultivateHQ/phoenix_guides,keathley/phoenix,trarbr/phoenix,Kosmas/phoenix,eksperimental/phoenix,mobileoverlord/phoenix,jwarwick/phoenix,greyhwndz/phoenix_guides,philss/phoenix_guides,Joe-noh/phoenix,3quarterstack/phoenix_guides,phoenixframework/phoenix,tianyi33/phoenix_guides,fblecha/phoenix_guides,Kosmas/phoenix,jwarwick/phoenix,optikfluffel/phoenix,bsmr-erlang/phoenix,bsmr-erlang/phoenix,nshafer/phoenix,pap/phoenix,nshafer/phoenix,mschae/phoenix,gjaldon/phoenix,Joe-noh/phoenix,clessg/phoenix_guides,phoenixframework/phoenix,stevedomin/phoenix,mitchellhenke/phoenix,phoenixframework-Brazil/phoenix_guides,mitchellhenke/phoenix,trarbr/phoenix,michalmuskala/phoenix,optikfluffel/phoenix,oldfartdeveloper/phoenix_guides,ugisozols/phoenix_guides