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.

I see lots of people specifying 2.1.1 and 2.1.2 in their Travis' config files but in my opinion they should just use 2.1.

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 (at this point 2.1.2) will be selected automatically.

I think what I wrote probably deserves to be added to the Ruby documentation for now I think these changes will do.

[1]:
https://www.ruby-lang.org/en/news/2013/12/21/ruby-version-policy-changes-with-2-1-0/