Qwt 6.1.0 upgrade is causing crashes or build problems
Downgrade to Qwt 6.0.1 and reinstall
$ brew remove qwt
$ git checkout 4392eba Library/Formula/qwt.rb
$ brew install qwt
QGIS shows SIP error messages on startup, indicating that QGIS is using the system Python instead of Homebrew's Python
Fix QGIS Python linking by using @jenshnielsen's qgis.rb
version (see PR #242)
$ cd Library/Taps/homebrew-science/
$ git pull https://github.com/jenshnielsen/homebrew-science.git qgis_python