Force installing the latest version of isort

Version 4.2.5 that is installed on Travis prints false positives causing
checks to fail.

Make sure that version 4.3.x or higher gets installed.

.travis.yml

@@ -24,7 +24,7 @@ install:
   - "pip install .[test]"
   - "pip install pylint"
   - "pip install codecov"
-  - "pip install isort"
+  - "pip install -U isort"
 script:
   - "green -vvr $GREEN_OPTS"
   - "python -m pylint --errors-only topydo test"