Bump mocha version to support Ruby 2.1

:information_desk_person: NB, this targets the `legacy-v2` branch.

Due to the version of `minitest` in Ruby's standard library changing in v2.1, Changing the version of `mocha` is required to get the test suite passing for that version.

```
bundle exec rake
/opt/rubies/ruby-2.1.2/bin/ruby -I"lib:lib:test" -I"/Users/s/.gem/ruby/2.1.2/gems/rake-10.3.2/lib" "/Users/s/.gem/ruby/2.1.2/gems/rake-10.3.2/lib/rake/rake_test_loader.rb" "test/**/*_test.rb" 
Run options: 

# Running tests:

[  1/618] AccurevTest#test_internal_revision_parse/Users/s/.gem/ruby/2.1.2/gems/mocha-0.9.12/lib/mocha/integration/mini_test/version_201_to_202.rb:19:in `run': uninitialized constant MiniTest::Unit::TestCase::SUPPORTS_INFO_SIGNAL (NameError)