Add bundler as a dependendy

We tried to run it in a vanilla vm with a brand new ruby installation and it seems it needs bundler as a dependency
```
/opt/rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- bundler (LoadError)
	from /opt/rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /opt/rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/bosh-bootstrap-0.13.2/lib/bosh-bootstrap/cli/helpers/bundle.rb:5:in `<top (required)>'
```

@bonzofenix && @lacciagioni