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
Convert method names to symbols before checking for existence

This prevents private callbacks from being mistakenly overlooked because
the strings passed from run_on_entry and run_on_exit don't compare as equal
to the symbols presented by self.private_methods.
Add note according to #126
Modify the documentation note

Now the text works for both upcoming release and released gem versions.
Bump version to 1.3.0
Merge pull request #125 from javawizard/convert-method-names-to-symbols

Convert method names to symbols before checking for existence
Merge pull request #118 from tmaier/retire-1-8-7

Retire Ruby 1.8.7
Ruby 1.8.7 and Rails 2.x retirement note; delete more obsolete stuff
Merge pull request #113 from virtualstaticvoid/master

Add support for specifying event `if` conditions with a method name symbol
Fix README for yard documentation generator
Fix #131 - add a more clear note regarding incompatibility to the Changelog.
More changelog documentation
Fix README markdown formatting