Get rid of the Gemfile.lock

[smortex]

The lockfile allow reproducible build, which are awesome. However, we also want to support multiple versions of Ruby, and a lockfile is unfortunately not a one-size-fit-all solution in this case:

• Older versions of bundler cannot run on recent versions of Ruby;
• Recent versions of bundler cannot run on older versions of Ruby;
• Some dependencies are available for some versions of Ruby but not all the ones we support, so a version of Ruby may be unable to run the bundle generated by another version of Ruby.

The lock file is only used when someone clone the repository and run bundle install. When installing with gem install riemann-dash (the way recommanded in the README), the dependencies only have to be in the bounds of what is authorized in the gemspec file, so this will only affect a fraction of our users which are probably aware of what they are doing.

This allows us to run the test suite on all supported versions of Ruby.