Reset some more ruby/gem env-variables before running "puppet"

Similar to #182 we're experiencing `puppet` not to run correctly,
because some ruby/gem environment is inherited from vagrant's embedded ruby.

This is happening now that librarian-puppet doesn't depend/use the gem's puppet anymore. 

Example error:
```
/Applications/Vagrant/embedded/gems/gems/bundler-1.5.3/lib/bundler/spec_set.rb:92:in `block in materialize': Could not find builder-3.2.2 in any of the sources (Bundler::GemNotFound)
```