Fix libv8 build fail on ARM (raspberry pi)

therubyracer 0.11.3 depends on libv8 ~> 3.11.8.12 that does not build on ARM (tested on raspberry pi).

This PR fixes that issue by depending on therubyracer 0.12.0 that depends on libv8 ~> 3.16.14.0 that builds on ARM (providing foreman works well with therubyracer 0.12.0).