Fix failure during powerline font patching

Since commit Homebrew/homebrew@0b50110, fontforge keg is installed without Python extension by default. 
Thus, this keg fails during powerline font [patching process](https://github.com/sanemat/homebrew-font/blob/1d196b1fa9b19862f6f9c64d7ffa751cbead4442/ricty.rb#L92). This pull request adds '--with-python' option to font forge build, which forces to build fontforge with Python extension enabled.