Symlink ruby.exe to ruby

Rails 4 introduced the `bin/` directory to generated projects with executables `rails`, `bundle`, `rake`, etc. The file generated on windows has a link to an incorrect `ruby.exe` in the shebang line. See #134.

This PR adds a symlink so that `ruby.exe` can be called in addition to `ruby.
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
update changelog
v78
remove debug output from LXPC

If someone is using user-env-compile and sets DEBUG to true, then the
output will be displayed causing a build failure.
Merge pull request #139 from heroku/schneems/rails4-windows-fix

Symlink ruby.exe to ruby
fix syntax error in spec
Merge pull request #141 from heroku/remove_debug_lxpc

remove debug output from LXPC
update changelog
v79
Remove RUBY_VERSION env var support

The docs says it was going to be removed entirely in August 2012.

It is now Sept 2013. About time to remove it.
Merge pull request #144 from heroku/remove_ruby_version_env_var

Remove RUBY_VERSION env var support