[merb-core] Fixing bundler integration
[merb-core] Fixing bundler integration

This patch fixes Bundler integration to be compatible with Bundler
versions 0.7.2 and 0.8.1.

We won't probably update Bundler integration until Bundler 1.0 is out
due to it's turbulent development.

merb/merb-core,merb/merb-core