Improve Homebrew and 1.9+ detection

The Homebrew workaround needed some tweaking and improvements, since apparently it is inconsistent as to whether it links ICU as /user/local/opt/icu4c or not.

Replaced the version detection for 1.9 with feature detection for the method in question. This makes charlock_holmes work on Ruby 2. Note: You have to require charlock_holmes before you include charlock_holmes/string if you want the String extensions to work on Ruby 2, for some as yet unknown reason.