Update tested Rubies

Changes include:

* Removing 1.8.7 from default config since it was end-of-life'd.

* Specifying Ruby 2.1 instead of 2.1.0.

  Since Ruby 2.1.0, the MRI versioning policy changed to follow Semantic
  Versioning [1].

  Thus, it is better to use '2.1' instead of '2.1.x' in the config file
  since this way the latest patch release will be selected automatically.

[1]: https://www.ruby-lang.org/en/news/2013/12/21/ruby-version-policy-changes-with-2-1-0/