Reverted rstan version to 2.17.1

Author: VisruthSK

DESCRIPTION

@@ -51,7 +51,7 @@ Suggests:
     loo (>= 2.0.0),
     RColorBrewer,
     rmarkdown (>= 1.0.0),
-    rstan (>= 2.36.0),
+    rstan (>= 2.17.1),
     rstanarm (>= 2.17.4),
     rstantools (>= 1.5.0),
     scales,