Added empty .gemtest to silence bundler warnings.

Bundler complains that the RubyGem isn't valid
with the following error message when installed
via Bundler from a Git repo.

Given a Gemfile with the following gem reference

gem 'text-hyphen', github: 'halostatue/text-hyphen'

`bundle install` generates:

Using text-hyphen (1.4) from git://github.com/halostatue/text-hyphen.git (at master)
text-hyphen at /<SNIPPED PATH>/bundler/gems/text-hyphen-b65fe6d29bd6 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  [".gemtest"] are not files