Follow the fix below if you get an error message like the following:
error: can't combine user with prefix, exec_prefix/home, or install_(plat)base
Fix: Then the simply omit the
--user option and your install will complete just fine.
See the information and links below for the source of the fix.
Note: the latest docs are incorrect as of 2014-03-04.
The correct instructions can be found in the changes on Github: Update OSX installation instructions - b988b1b - davidcelis/powerline - homebrew powerline installation instructions.