CI: Don't install Sphinx if Python 2

Recently builds started to fail with:

    Collecting sphinx
      Downloading Sphinx-1.7.2-py2.py3-none-any.whl (1.9MB)
        100% |████████████████████████████████| 1.9MB 731kB/s
    Sphinx requires Python '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
    but the running Python is 2.6.9

.travis.yml

@@ -9,8 +9,9 @@ python:
   - 3.6
   - nightly
 install:
-  - pip install pyyaml flake8 flake8-import-order coveralls sphinx
+  - pip install pyyaml flake8 flake8-import-order coveralls
   - if [[ $TRAVIS_PYTHON_VERSION == 2.6 ]]; then pip install unittest2; fi
+  - if [[ $TRAVIS_PYTHON_VERSION != 2* ]]; then pip install sphinx; fi
   - pip install .
 script:
   - if [[ $TRAVIS_PYTHON_VERSION != 2.6 ]]; then flake8 .; fi