.travis.yml: Pin isort at 3.9.0

isort is a tool that checks that imports are sorted alphabetically and
grouped in a PEP8-compliant way. Somewhere between isort 3.9.0 and 3.9.4
a change was introduced that breaks our `make check`: Uppercase letters
now sort before all lowercase letters.
Merge pull request #243 from drothlis/pin-isort

.travis.yml: Pin isort at 3.9.0