Removed custom CFLAGS from Travis config file to better reflect working env.
Removed custom CFLAGS from Travis config file to better reflect working env.

Using `-O0` gcc flag improves build time but has a negative impact on numeric
precision and makes some tests fail.

rgommers/pywt,nigma/pywt,ThomasA/pywt,ThomasA/pywt,eriol/pywt,ThomasA/pywt,Dapid/pywt,michelp/pywt,michelp/pywt,kwohlfahrt/pywt,Dapid/pywt,grlee77/pywt,grlee77/pywt,kwohlfahrt/pywt,aaren/pywt,aaren/pywt,nigma/pywt,Dapid/pywt,PyWavelets/pywt,eriol/pywt,rgommers/pywt,rgommers/pywt,rgommers/pywt,nigma/pywt,PyWavelets/pywt,kwohlfahrt/pywt,eriol/pywt,michelp/pywt,aaren/pywt