try fixing travis build

it seems that, for whatever reason, the issue described here has resurfaced:
aiidateam#2420 (comment)

It was thought that this could be prevented by adding the
`--no-cache-dir` argument to pip here:
aiidateam#2799

but apparently this fix was fortuituous?

.travis.yml

@@ -36,6 +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 install numpy==1.15.4
     # Install AiiDA with some optional dependencies
     - if [ "$TEST_TYPE" == "docs" ]; then pip install --no-cache-dir . && pip install --no-cache-dir -r docs/requirements_for_rtd.txt; else pip install .[all]; fi