TravisCI: Fix sphinx invocation
TravisCI: Fix sphinx invocation

When installed through pip rather than apt-get, the sphinx-build command does not go to the same location. Moreover, there are not separate commands created for Python versions 2 and 3. Therefore, instead invoke sphinx through the relevant Python interpreter.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3