Remove rubygems from list of Rake requirements.

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

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

Increment the minor version to indicate:

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