Remove support for ruby 1.8 (fixes #683)

1.8 cruft removed!

* removed 1.8-only methods
* updated comments and README
* removed 1.8.7 from Travis
* added 1.9.2 to Travis
* required ruby >= 1.9.2 in gemspec

TODO: 
* bump version. To comply with semantic versioning, this should be part of a 5.0 release.
* add a post install message to the gemspec, similar to the 4.0 message

I can re-submit to a 5.0 branch if that makes it easier to manage the current version and future release.