(maint) Add Ruby 2.0.0 to Travis build matrix

Without this patch we're not testing against Ruby 2.0.0 which has recently been
released.  This is a problem because we'd like a way to be notified if a change
set breaks compatibility with future supported versions of Ruby.

This patch should not be taken as an indication that we fully support Ruby 2.0,
just as an indication that we plan to in the future.