Run ProtectedBranches integration test on Git 2+

The tests fail on Git 1.8, but it's not worth the effort to debug at
this point in time. Should someone complain in the future, we can
investigate further.

Author: sds

spec/integration/protected_branches_spec.rb

@@ -1,6 +1,7 @@
 require 'spec_helper'
 
-describe Overcommit::Hook::PrePush::ProtectedBranches do
+describe Overcommit::Hook::PrePush::ProtectedBranches,
+         if: Overcommit::GIT_VERSION >= '2.0' do
   let(:flags) { '' }
   let(:pushed_ref) { remote_ref }
   subject do