Feature/gemspec dependencies should use pessimistic operator

Updates the gemspec to use the RubyGems pessimistic operator. [1][2]

Bumps the gem version to 0.10.0.pre7.

#### Run Loop

Depending on Run Loop >= 1.0.0.pre5 - needs to be changed before release.

#### Bundler

I think we should consider moving bundler to the development dependencies.  I agree that we should be encouraging everyone to use bundler.  Bundler is not (yet) a runtime requirement and it is very easy to get out of step with xamarin-test-cloud's bundler dependencies. [3]

#### Running Tests Locally

```
$ script/ci/travis/local-run-as-travis.rb
$ script/ci/travis/xtc-submit-ci.rb
```

#### Links

- [1] http://guides.rubygems.org/patterns/#semantic-versioning
- [2] http://robots.thoughtbot.com/rubys-pessimistic-operator
- [3] https://github.com/xamarin/test-cloud-command-line/blob/master/xamarin-test-cloud.gemspec