Updated Yara to version 2.1

Yara 2.0 had some major changes to the functionality of Yara and changed the build process quite a bit. I updated the binary, changed the build, and fixed the dependencies. 

The formula technically runs configure twice, once when running the included ```build.sh``` script and once when reconfiguring with the Homebrew options, but frankly it worked and I figure there are worse things than that.