Lock to bundler 1.7.7 to avoid warnings.

Our specs that check there are no load-time warnings are failing
due to a warning coming from bundler:

C:/Ruby193/lib/ruby/gems/1.9.1/gems/bundler-1.7.8/lib/bundler/source/rubygems.rb:191: warning: shadowing outer local variable - spec

I think 1.7.7 does not have this error.