Always install newest pytest
Always install newest pytest

This will hopefully fix the build failures we are seeing in Travis,
which seem to be caused due to an incompatibility between the version of
pytest required by pytest_cov and the actual version that is already
installed in the virtual environment.

Signed-off-by: Rodrigo Tobar <bd955c861cbec2e41c0959cf13c7f573fa307e58@icrar.org>

steve-ord/daliuge,steve-ord/daliuge,steve-ord/daliuge,steve-ord/daliuge,steve-ord/daliuge,steve-ord/daliuge,steve-ord/daliuge