Update .travis.yml

Tested it this time

.travis.yml

@@ -16,7 +16,7 @@ install:
   - pip install .
 script:
     # https://github.com/PyCQA/pylint/issues/1113
-  - if [[ $TRAVIS_PYTHON_VERSION != 3.6 ]] && [[ $TRAVIS_PYTHON_VERSION != nightly]]; then pylint --rcfile .pylintrc -E rtv/; fi
+  - if [ $TRAVIS_PYTHON_VERSION != 3.6 ] && [ $TRAVIS_PYTHON_VERSION != nightly ]; then pylint --rcfile .pylintrc -E rtv/; fi
   - coverage run -m py.test -v
 after_success:
   - coveralls