new numpy version

.travis.yml

@@ -36,7 +36,7 @@ before_install:
 install:
     # Upgrade pip setuptools and wheel to be able to run the next command
     - pip install -U pip==18.1 wheel setuptools coveralls
-    #- pip uninstall --yes numpy # try fixing problems with pre-existing numpy version
+    - pip install numpy==1.15.4 # try fixing problems with pre-existing numpy version
     - pip freeze
     # 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