Added Gemfile.lock to .gitignore.
Removed ZenTest from Gemfile.

Attempting to bundle with ZenTest results in errors like the following:

Invalid gemspec in [/home/travis/.rvm/gems/ruby-1.9.3-p327/specifications/ZenTest-4.8.4.gemspec]: Illformed requirement ["< 2.1, >= 1.8"]

See eg https://travis-ci.org/brixen/shen-ruby/jobs/4917041