Update minimum Ruby version to 1.9.3, add travis-ci support for 2.0.0

Update:
 * gemspec's minimum Ruby version to >= 1.9.3
 * .ruby-version to use 2.0.0 (newest supported ruby version)

Remove:
 * Ruby 1.9.2 testing for travis-ci

Add:
 * Ruby 2.0.0 testing for travis-ci