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

.travis.yml

@@ -1,6 +1,5 @@
 language: ruby
 rvm:
-  - 1.9.2
   - 1.9.3
   - 2.0.0
   - jruby-head