[merb-core] More robust dependency loading
[merb-core] More robust dependency loading

Fixes bug where merb-gen wasn't able to generate
any apps anymore.

Tries to load the dependencies like this:

1.  Use Merb::Config[:gemfile] or default Gemfile
2a. Try to require an already bundled environment
2b. If a Gemfile is present, use bundler to load it
2c. Warn if no Gemfile can be found

merb/merb,merb/merb,merb/merb