Drop Ruby 1.8 support.

In June 2013, Ruby 1.8.7 will officially reach end-of-life (EOL) status 
meaning that *no* more patches will be released. Additionally, Ruby Enterprise
Edition (REE) has reached EOL. JRuby and Rubinius both have 1.8 and 1.9 modes
so there are upgrade paths available here.

Finally, relying on a Ruby version equal or greater to 1.9 eliminates the
[multi_json requirement](https://github.com/intridea/multi_json/pull/113#issuecomment-17668823) which has started throwing warnings when the stdlib
version of json is being used.

Users requiring Ruby 1.8 can continue to use the 0.1.x gem releases which
remain functional and highly stable.