Specify RSpec version

Hello,

I specified the version of RSpec in `gitpusher.gemspec` explicitly because
too old version(2.0.1) are installed without it today and the version foil tests.

Though I don't know exact reason why, today, Bundler installs 2.0.1 because
Gitpusher depends on Grit, Grit depends on Diff::LCS ~> 1.1, and Diff::LCS depends on rspec ~> 2.0.

I tried some versions of RSpec and found that version 2.11.0 and 2.12.0 are okay.