Work around pypa/virtualenv#1555

.travis.yml

@@ -72,21 +72,21 @@ before_install:
     fi
 
 install:
-  - pip install -U pip setuptools
-  - pip install -U coveralls coverage
+  - python -m pip install -U pip setuptools
+  - python -m pip install -U coveralls coverage
   # pip install zope.proxy here because because setup_requires will use
   # easy_install, which fails on Python 3.5 on Mac OS
-  - pip install -U zope.proxy
-  - pip install -U -e .[test,docs]
+  - python -m pip install -U zope.proxy
+  - python -m pip install -U -e .[test,docs]
 
 script:
   - python --version
-  - coverage run -m zope.testrunner --test-path=src
-  - coverage run -a -m sphinx -b doctest -d docs/_build/doctrees docs docs/_build/doctest
+  - python -m coverage run -m zope.testrunner --test-path=src
+  - python -m coverage run -a -m sphinx -b doctest -d docs/_build/doctrees docs docs/_build/doctest
   - python setup.py bdist_wheel
 
 after_success:
-  - coveralls
+  - python -m coveralls
   - |
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       # macpython 3.5 doesn't support recent TLS protocols which causes twine