Downgraded resolver due to bug in the latest version of stack.

The latest version of stack has a small incompatibility with the
latest version of cabal.
Refer to commercialhaskell/stack#3791

Author: gahag

stack.yaml

@@ -15,7 +15,7 @@
 # resolver:
 #  name: custom-snapshot
 #  location: "./custom-snapshot.yaml"
-resolver: lts-10.3
+resolver: lts-9.21
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.
@@ -63,4 +63,4 @@ packages:
 # extra-lib-dirs: [/path/to/dir]
 #
 # Allow a newer minor version of GHC than the snapshot specifies
-# compiler-check: newer-minor
+# compiler-check: newer-minor