Don't assume RUBY_PATCHLEVEL exists

Apparently, on very old Ruby (1.8.5), this doesn't exist; see:

  https://github.com/wincent/Command-T/issues/104

I can't test this as I don't currently have access to a machine with
1.8.5 on it, but it seems a reasonable change to make.
Attempt fix for build breakage on old Ruby

Reported here:

  https://github.com/wincent/Command-T/issues/104

`ruby extconf.rb` was blowing up with:

  extconf.rb:24: undefined method `+' for nil:NilClass (NoMethodError)

Alas, I can't test this, but I'll push this out to the "next" branch
anyway.
doc: mention Ruby 1.8.5 compatibility fixes in the HISTORY section