fix "uninitialized constant Gem::UserInteraction" error in webui

On older version of rubygems, I get the following error when trying to visit the geminabox web ui:
```        
uninitialized constant Gem::UserInteraction
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/indexer.rb:16:in `<class:Indexer>'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/indexer.rb:14:in `<top (required)>'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /usr/local/rvm/gems/ruby-1.9.3-p125/gems/geminabox-0.11.0/lib/geminabox.rb:5:in `<top (required)>'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `require'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
from /usr/local/rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
from config.ru:2:in `block in <main>'
```

This problem occurs on several versions of rubygems that I tested: 1.8.19 and 1.8.25, but does not occur on 2.0.7.  The attached fix makes it works in all versions now.