Set up Travis to run the python tests. (#160)

* Set up Travis to run the python tests.

* np.matmul requires Numpy 1.10.0

* Extract python dependencies into requirements.txt

.travis.yml

@@ -0,0 +1,11 @@
+language: python
+python:
+  - "2.7"
+  - "3.6"
+
+install:
+  - pip install --upgrade pip
+  - pip install -U -r python/requirements.txt
+
+script:
+  - cd python && python setup.py develop test

python/requirements.txt

@@ -0,0 +1,5 @@
+Cython>=0.22
+pystan>=2.14
+numpy>=1.10.0
+pandas>=0.18.1
+matplotlib>=2.0.0