Remove travis CI workaround

This is no longer necessary since runtests was recently fixed
to not spawn multiple parallel pytest jobs.

Author: JukkaL

.travis.yml

@@ -16,5 +16,5 @@ install:
   - python setup.py install
 
 script:
-  - python runtests.py -x lint -j4
+  - python runtests.py -x lint
   - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then flake8; fi