This gem can be a PITA to install on a mac, this is what worked for me. If you get this error:
First, make sure you have installed the required dependencies:
brew install xz icu4c
Now, install charlock_holmes:
--with-opt-dir=/usr/local/opt --with-opt-include=/usr/local/opt/icu4c/include --with-opt-lib=/usr/local/opt/icu4c/lib --with-cxxflags=-std=c++11
