Update mongrel2 to version 1.9.1

Version 1.8.1 is outdated and has major bugs that are fixed in 1.9.1, which is a stable release version.

Version 1.9.1 also has support for ZeroMQ, which obsoletes the patch required beforehand. Some versions of OSX wont build with `ENV.ldflags` by itself because the project depends on a library that doesn't export it's symbols, so the added ldflag allows it to build properly.