Reset all bundler, ruby and gem env vars before calling puppet

Followup from https://github.com/rodjek/librarian-puppet/pull/185
Unfortunately it wasn't enough :/

I was again getting mixed stack traces between the system's and vagrant's rubygems.. 
```
/Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/spec_set.rb:92:in `block in materialize': Could not find diff-lcs-1.2.5 in any of the sources (Bundler::GemNotFound)
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/spec_set.rb:85:in `map!'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/spec_set.rb:85:in `materialize'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/definition.rb:133:in `specs'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/definition.rb:178:in `specs_for'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/definition.rb:167:in `requested_specs'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/environment.rb:18:in `requested_specs'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/runtime.rb:13:in `setup'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler.rb:119:in `setup'
	from /Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/setup.rb:17:in `<top (required)>'
	from /usr/local/Cellar/ruby/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Cellar/ruby/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
```