unable to bundle HEAD rails master

Seems to be a problem bundling current rail master (7a32c6300c2b11dc1660338535c653a0132df196)

workaround is removing the arel requirement from Gemfile

seems this is because rails gemspec specified arel and master of arel are incompatible version:

    Bundler could not find compatible versions for gem "arel":
      In Gemfile:
        rails (>= 0) ruby depends on
          arel (~> 3.0.2) ruby

        arel (4.0.0.beta2.20130314230643)

https://github.com/carlhuda/bundler/issues/2419