Relax mime-types requirement to allow v2.

This PR fixes #641.

I ran the specs on MRI 2.0.0 on Linux (Fedora 20 x86_64, if it matters) and the spec result is the same for mime-types >~ 1.16 and mime-types >= 1.16 (which resolves to 2.1 on my machine).

The main advantage of this approach, over straight-up moving to 2.0+, is that folks who need MRI 1.8 can still set the mime-types requirement in their Gemfile.

Thank you very much for the mail gem!
Please consider merging this so you can make life easier for folks who need / want mime-types 2!