Install packages needed for testing docs

.travis.yml

@@ -54,6 +54,7 @@ install:
   - if [ $UNCERTAINTIES == 'Y' ]; then pip install uncertainties; fi
   - if [ $NUMPY_VERSION != '0' ]; then conda install -c mwcraig --yes numpy==$NUMPY_VERSION; fi
   - if [ $TRAVIS_PYTHON_VERSION == '3.2' ]; then pip install coverage==3.7.1; else pip install coverage; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == '3.5' && $NUMPY_VERSION == 1.10.4 && $UNCERTAINTIES == "Y" ]]; then pip install serialize pyyaml; fi
   - pip install coveralls
 
 script:

pint/testsuite/__init__.py

@@ -123,8 +123,14 @@ def testsuite():
     """
     suite = unittest.TestLoader().discover(os.path.dirname(__file__))
     from pint.compat import HAS_NUMPY, HAS_UNCERTAINTIES
-    if HAS_NUMPY and HAS_UNCERTAINTIES and sys.version_info[0:2] == (3, 5):
-        add_docs(suite)
+
+    # TESTING THE DOCUMENTATION requires pyyaml, serialize, numpy and uncertainties
+    if HAS_NUMPY and HAS_UNCERTAINTIES:
+        try:
+            import yaml, serialize
+            add_docs(suite)
+        except ImportError:
+            pass
     return suite