Specify Ruby 2.1 in config 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/