Module#ancestors on singleton class changed from Ruby 2.0 to 2.1

The behaviour of `Module#ancestors` on a singleton class changed [1,2] between
Ruby v2.0 and v2.1 so that it now includes the singleton class itself.

This commit fixes #5 and hopefully fixes the Travis CI build.

[1] https://bugs.ruby-lang.org/issues/8035
[2] https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/39628/diff/