Adds support for Travis CI.

-Add travis configuration file (tests rubies 1.8.7, 1.9, 2.0, and rbx 1.9 mode).
-Fixes for unit tests breaking under Ruby 1.8.7.
-Updated gemspec to add check for signing key so that we can also build unsigned gem.
-Add travis build status image to README.rdoc.
Fix Memcached::Rails write to allow nil options to be passed.
Merge remote-tracking branch 'origin/memcached-rails-fix-write-with-nil-options' into memcached-rails-updates
The default options for fetch should be nil.
Updated CHANGELOG and bumped version.
Merge pull request #133 from joe1chen/memcached-rails-updates

Memcached rails updates