should set $GEM_PATH as well for the build process

if this isn't setup properly, locally it might pick the wrong bundler
Bugfix and More tests for plugin logic

Actually testing for presence of directories now !
rails 4 plugin work

* move rails_12factor gem test to the beginning
* test against ruby 2 since that's the new default
* rails_12factor not rails_12_factor
rails 3 plugin work

* gem and plugin names are different
* rails_12factor not rails_12_factor
only warn once about plugin injection in rails 3
fix specs
use `PluginInstaller` for runtime asset compilation
set deprecation notices for runtime assets compilation
fix specs