Fix bootstrap process duplicate gems and maven issue

* During `rake bootstrap` or `rake artifact:tar` gems were being installed more than once since the GEM_PATH environment variable wasn't set to cover all sources of gem directories.
* pinning maven-tools to 1.0.5 works around https://github.com/torquebox/maven-tools/pull/7 
* move plugins Gemfile to `tools/Gemfile.plugins`