Drops Python 3.3 from .travis.yml.

Pip no longer supports Python 3.3:
pypa/pip#3796

Python 3.3's end of life was roughly a year ago anyway, so
this seems like a good time to drop it from the test matrix:
https://www.python.org/dev/peps/pep-0398/#x-end-of-life

.travis.yml

@@ -12,8 +12,6 @@ matrix:
     env: TOX_ENV=py27-oauth2client3
   - python: "2.7"
     env: TOX_ENV=py27-oauth2client4
-  - python: "3.3"
-    env: TOX_ENV=py33-oauth2client4
   - python: "3.4"
     env: TOX_ENV=py34-oauth2client4
   - python: "3.5"
@@ -25,8 +23,6 @@ matrix:
   - python: "3.5"
     env: TOX_ENV=py35-oauth2client4
 install:
-  # See https://github.com/aws/base64io-python/issues/4#issuecomment-397857642
-  - if [[ $TRAVIS_PYTHON_VERSION == 3.3 ]]; then pip install virtualenv==15.2.0; fi
   - pip install tox
   - pip install . argparse
 script: tox -e $TOX_ENV