update commit reference for stan repo to v2.17.1 (#404)

ariddell · web-flow · commit 7d32df805bf1 · 2018-01-07T08:28:57.000-05:00
diff --git a/pystan/stan b/pystan/stan
@@ -1 +1 @@
-Subproject commit 52d23a6f8365df67b5402c975bf4299c20f0966a
+Subproject commit b672017bc2748b2962126859662b658aa9f9bba6
