Several Changes

This PR contains several changes

* Since the current rspec issues warnings about using the old-style matchers, I refactored the spec to use the expect syntax.
* Somehow sql_parser/version was being loaded before sql_parser.treetop when running rspec, even though nowhere is sql_parser/version explicitly loaded. So I inlined the version constant in the gemspec.
* Made keywords do a case-insensitive match. I found no better way than to use `[kK] [eE] [yY] [wW] [oO] [rR] [dD]`, let me know if there is an easier way.