Bump ruby to 2.1.2

This requires a change in a test because in ruby 2.1.2, trying to modify
a frozen class raises a RuntimeError, rather than a TypeError.

The main driver for this upgrade is test execution speed. On my dev VM:

ruby 2.1.2
real	0m26.137s
user	0m16.349s
sys	0m5.038s

ruby 1.9.3
real	0m46.030s
user	0m34.183s
sys	0m6.883s