Bump dependency version

Hi. I noticed current released omnifocus.rb is 2.1.x and there are several improvements and fixes made in 2.1.x. However, omnifocus-pivotaltracker's dependency pattern ( http://guides.rubygems.org/patterns/ ) prevents omnifocus-pivotaltracker from working with 2.1.x with current spec (btw, 2.1.0 was released back in 2012 https://github.com/seattlerb/omnifocus/commit/0ec3e0a3bf7b ).

There could be lots of methods to work with this, but I guess putting base version as 2.1.3 and keeping up with minor updates (in 2.1.x) would be enough this time.