Fixing dependencies for API compatibility

Proposed fix for Issue #86

Current cucumber-nagios is not yet compatible with the latest versions
of cucumber, mechanize and amqp. Limiting version dependencies to known
compatible ones for now, until further development takes place.

I've committed changes to Gemfile.lock, though some suggest it shouldn't
be included in the repo for a gem.
(ref http://stackoverflow.com/a/4151540/418602)