Install S3 dependencies in CI

Enables the testing of the S3 persister during CI.

.travis.yml

@@ -23,7 +23,7 @@ install:
   - conda install -q python=$TRAVIS_PYTHON_VERSION
   - pip install -r requirements.txt -r requirements-dev.txt
   - pip install pandas==0.23.4  # for rpy2 compatibility
-  - pip install -e .[docs,testing]
+  - pip install -e .[docs,testing,S3]
 script:
   - travis_wait py.test --runslow
 deploy: