Travis: fix NumPy 1.14 install

.travis.yml

@@ -21,7 +21,8 @@ python:
 # Install packages
 install:
   - pip install --upgrade pip setuptools wheel
-  - pip install -r requirements.txt --only-binary=numpy,scipy numpy==1.14
+  - pip install --only-binary=numpy numpy==1.14
+  - pip install --only-binary=scipy -r requirements.txt
   - pip install coveralls
   - pip freeze