Bump Ruby version to 1.9.3

OS X 10.8 doesn't have a build, and for some reason RVM doesn't want to build it from source:

	$ rvm install ruby-1.9.2-p320
	No binary rubies available for: osx/10.8/x86_64/ruby-1.9.2-p320.
	Continuing with compilation. Please read 'rvm mount' to get more information on binary rubies.
	The provided compiler '/usr/bin/gcc' is LLVM based, it is not yet fully supported by ruby and gems, please read `rvm requirements`.

1.9.3 seems to work just fine so far.