make check: check pep8 import order only if `isort` is installed
make check: check pep8 import order only if `isort` is installed

`isort` isn't installable with yum/apt-get on Fedora, Debian or Ubuntu;
some developers find it a bit annoying to be forced to use pip, so make
this optional. The important thing is that Travis will check it.

LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester