Retire Ruby 1.8.7

https://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/
Fix inheritance test
Replace eigenclass with define_singleton_method
Fix for Ruby 1.9.3
Set required_ruby_version to 1.9.2 in gemspec
Merge pull request #118 from tmaier/retire-1-8-7

Retire Ruby 1.8.7