add sklearn to travis yml

Author: ievred

.travis.yml

@@ -35,6 +35,7 @@ install:
   - pip install -r requirements.txt
   - pip install -U "numpy>=1.14" "scipy<1.3"  # for numpy array formatting in doctests + scipy version: otherwise, pymanopt fails, cf <https://github.com/pymanopt/pymanopt/issues/77>
   - pip install flake8 pytest "pytest-cov<2.6"
+  - pip install -U "sklearn"
   - pip install .
 # command to run tests + check syntax style
 services: