MAINT: Cache travis-ci pip install
MAINT: Cache travis-ci pip install

Cache pip install on travis-ci as well.

rgommers/pywt,rgommers/pywt,rgommers/pywt,eriol/pywt,rgommers/pywt,PyWavelets/pywt,eriol/pywt,grlee77/pywt,PyWavelets/pywt,kwohlfahrt/pywt,kwohlfahrt/pywt,michelp/pywt,grlee77/pywt,eriol/pywt,kwohlfahrt/pywt,michelp/pywt,michelp/pywt