Call pip install with the --no-cache-dir option

This prevents a mismatch between for example `numpy` and `scipy`.

.travis.yml

@@ -50,7 +50,7 @@ install:
     - pip install -U pip wheel setuptools
     - pip install coveralls
     # Install AiiDA with some optional dependencies
-    - if [ "$TEST_TYPE" == "docs" ]; then pip install . && pip install -r docs/requirements_for_rtd.txt; else pip install .[all]; fi
+    - if [ "$TEST_TYPE" == "docs" ]; then pip install . && pip install -r docs/requirements_for_rtd.txt; else pip install --no-cache-dir .[all]; fi
 
 env:
     ## Build matrix to test both backends, and the docs