update pip to use wheel binaries for numpy and scipy

See issue travis-ci/travis-ci#2650 for the suggestion to update pip and
request the wheels for scipy and numpy.

travis-ci/travis-ci#2650 (comment)

.travis.yml

@@ -5,7 +5,20 @@ python:
   - "3.5"
   - "3.6"
 
-install: 
+install:
+  # Travis has pre-installed numpy and scipy for Python 3.5 & 3.6, but not 3.4
+  # the following lines allow older Python & pip versions to use the binary
+  # wheels for numpy and scipy.  See: 
+  #      https://github.com/travis-ci/travis-ci/issues/2650#issuecomment-252285413
+  #      https://github.com/travis-ci/travis-ci/issues/2650#issuecomment-266721196
+
+  # Question: Do these two lines slow down the runs for Python 3.5 & 3.6?
+  #     It takes about 1 min 10 seconds to run Python 3.5 & 3.6 tests without adding these two pip install lines.
+  #     Pip uses wheel binaries already during the Python 3.5 & 3.6 runs.
+
+  - pip install --upgrade pip setuptools wheel
+  - pip install --only-binary=numpy,scipy numpy scipy
+
   # Install codecov so I can use it (and coverage.py) to run setup.py
   # However, keep codecov in setup.py so that it gets installed for local usage too.
   - pip install codecov