Drop test support for ruby 1.9.2

1.9.2 isn't supported by rails 4, and is difficult to test with travis
ci.

Attn @zmillman: I'd welcome further patches to better test ruby 1.9.2
support without compromising testing on 1.9.3 & 2.0.

See also: #21
[travisci] Add tests for activesupport 3.2 and 4.0