add site-packages tests to the travis build matrix (#2183)

Add `with_system_site_packages` variants of the two installed pythons (2.7, 3.4) so any plugin requiring pygobject/gstreamer can be tested with the CI system.

[appveyor skip]

.travis.yml

@@ -11,8 +11,12 @@ matrix:
     include:
         - python: 2.7
           env: {TOX_ENV: py27-cov, COVERAGE: 1}
+        - python: 2.7_with_system_site_packages
+          env: {TOX_ENV: py27-test}
         - python: 3.4
           env: {TOX_ENV: py34-test}
+        - python: 3.4_with_system_site_packages
+          env: {TOX_ENV: py34-test}
         - python: 3.5
           env: {TOX_ENV: py35-test}
         - python: 3.6-dev
@@ -51,7 +55,13 @@ install:
     - travis_retry pip install tox sphinx
     - travis_retry tox -e $TOX_ENV --notest
 
-script: tox -e $TOX_ENV
+script:
+    # prevents "libdc1394 error: Failed to initialize libdc1394" errors
+    - sudo ln -s /dev/null /dev/raw1394
+    - if [[ $TRAVIS_PYTHON_VERSION == *_site_packages ]]; then SITE_PACKAGES=--sitepackages; fi
+    # pip in trusty breaks on packages prefixed with "_". See https://github.com/pypa/pip/issues/3681
+    - if [[ $TRAVIS_PYTHON_VERSION == 3.4_with_system_site_packages ]]; then sudo rm -rf /usr/lib/python3/dist-packages/_lxc-0.1.egg-info; fi
+    - tox -e $TOX_ENV $SITE_PACKAGES
 
 # Report coverage to codecov.io.
 before_install: