use Ruby 1.9+ hash syntax, remove unneeded #to_s calls

Ruby 1.8 is EOLed, and the README says it's developed on 1.9, so this switches to 1.9+ hash syntax with symbols.

It also removes a few unneeded #to_s calls. Ruby's string interpolation with braces does this for you implicitly. It's actually also safer than +-based interpolation, which will throw errors if you haven't already #to_s'ed the code.