Now that support for earlier Rubies have been dropped, one can rely on
the presence of rubygems without an explicit require statement.

Rake learns to build, install, and release the Maildir gem.

Ensure copyright date includes Aaron's contributions from 2010 through
2014.

See <ktheory#17 (comment)>
for reference.

- Ruby 1.9.3 was made the minimum version way back in commit
  639fdb4.
- Link to the current version of the Travis-CI configuration file as the
  canonical reference to currently-supported Ruby versions.

Increment the minor version to indicate:

  1. Support for Ruby 1.8 and 1.9 has been dropped.
  2. Unit tests have been refactored to support Ruby 2.x and MiniTest
     5.x.