Revert "Removes support for Ruby < 2.0"

This reverts commit 9db4172090adc39c8c8d213f75a7ecfe2049ff01.

Conflicts:
	lib/classy_enum/base.rb

After thinking about this for a bit and looking at some of the recent
surveys on Ruby usage, it seems like a lot of people are still using
1.9.3. Since there's not a huge benefit to dropping support for 1.9.3,
it will stay for now.
Updates spec to ensure the enum option is covered

I thought about removing this in favor of class_name, but it's been
around for so long that it will cause some upgrade headaches for people
without much benefit.