Drop ruby 1.8.7 and rails 2.3 support

[andrew]

Both of these are pretty damn old now, I think it would be a good time to remove some of the old hacks for supporting them.

If you're still stuck on an old version v0.8.0 will still work just fine but future versions may not but split is pretty stable now so I can't see any other big changes coming in the near future.

Todo list:

• Remove readme comments on old version support
• Add readme note about supported versions
• Add note about supported versions to contributing.md
• Remove 1.8.7 has warning here: https://github.com/andrew/split/blob/master/lib/split/helper.rb#L5-L7
• add required_ruby_version '>= 1.9.2' to gemspec
• remove checks for rails > 3: https://github.com/andrew/split/blob/master/lib/split.rb#L18
• drop 1.8 json gemspec check: https://github.com/andrew/split/blob/master/split.gemspec#L26

[qpowell]

I'd be glad to help with this :)

[andrew]

@qpowell great! I don't mind if you want to do it as one big pull request or a number of little ones, once all the checkboxes are ticked then we can release it to rubygems as version 1.0.0.

[qpowell]

@andrew Cool, I should have some time to do this this week. I'll hit you up if I need any additional clarification :)

[andrew]

@qpowell perfect, no rush and yeah I'm happy to help out on any issues you run into. Feel free to point out any other weird/old/cruft bits in the codebase that I've become blind to.