Update setup.py

including -msse3 to solve the  #error “SSE2 instruction set not enabled” when installing scikit-bio via pip
This issue has been asked and adressed here: http://stackoverflow.com/questions/26211814/error-sse2-instruction-set-not-enabled-when-installing-scikit-bio-via-pip
The solution to incluede the -msee3 flag was found here: https://github.com/rmcgibbo/mixtape/issues/35