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.
Removes 1.8.7 from tested versions in README.
Stops handling 1.8.7 missing RUBY_ENGINE constant.
Merge pull request #108 from philnash/ruby-version

Bumps required ruby version to 1.9.3.