Bumps required ruby version.

Now we're using ruby 1.9 hash syntax, the gem should not report that the required ruby version starts at 1.8.7. I've put this up to 1.9.3 as it is the lowest supported version of ruby.

I'm not sure whether rubygems knows to use this version to not install gems on unsupported rubies, but if it does then we should probably yank the 3.13.0 gem as it won't work with 1.8.7 but advertises that it does.