Add Travis test matrices for pystan 2.17 and latest (#362)

.travis.yml

@@ -4,6 +4,10 @@ python:
   - "3.6"
   - "3.5"
 
+env:
+  - PYSTAN_VERSION=2.17
+  - PYSTAN_VERSION=latest
+
 addons:
   apt_packages:
     - pandoc

scripts/create_testenv.sh

@@ -9,9 +9,10 @@ command -v conda >/dev/null 2>&1 || {
 
 # if no python specified, use Travis version, or else 3.6
 PYTHON_VERSION=${PYTHON_VERSION:-${TRAVIS_PYTHON_VERSION:-3.6}}
+PYSTAN_VERSION=${PYSTAN_VERSION:-latest}
 
 if [[ $* != *--global* ]]; then
-    ENVNAME="testenv${PYTHON_VERSION}"
+    ENVNAME="testenv${PYTHON_VERSION}_PYSTAN${PYSTAN_VERSION}"
 
     if conda env list | grep -q ${ENVNAME}
     then
@@ -31,6 +32,12 @@ else
     pip install http://download.pytorch.org/whl/cpu/torch-0.4.1-cp36-cp36m-linux_x86_64.whl
 fi
 
+if [ "$PYSTAN_VERSION" = "latest" ]; then
+    pip install pystan
+else
+    pip install pystan==${PYSTAN_VERSION}
+fi
+
 conda install --yes numpy cython scipy pandas matplotlib pytest pylint sphinx numpydoc ipython xarray netcdf4 mkl-service
 
 pip install --upgrade pip