extract the bin/compile command in compile_spec
extract the bin/compile command in compile_spec

preparing for more invocations of bin/compile, and I don't want to keep
repeating myself.

SaberZaabouti/myBuildpack,SaberZaabouti/myBuildpack,dhaywael/static-buildpack,SaberZaabouti/myBuildpack,cloudfoundry/staticfile-buildpack,piyushcoolkarni/cfbuildpack,SaberZaabouti/myBuildpack,sks/staticfile-buildpack,cloudfoundry/staticfile-buildpack,cloudfoundry/staticfile-buildpack,piyushcoolkarni/cfbuildpack,jockhip12/mybuildpack,jockhip12/mybuildpack,cloudfoundry/staticfile-buildpack,piyushcoolkarni/cfbuildpack,sks/staticfile-buildpack,dhaywael/static-buildpack,SaberZaabouti/myBuildpack,sks/staticfile-buildpack,jockhip12/mybuildpack,jockhip12/mybuildpack,dhaywael/static-buildpack,piyushcoolkarni/cfbuildpack,jockhip12/mybuildpack,dhaywael/static-buildpack,dhaywael/static-buildpack,sks/staticfile-buildpack