Have latest transformers library installed.

This is important since some instances are included in this package. For
instance, MaybeT is not an instance of MonadTrans in the previous
package.

postlude.cabal

@@ -19,4 +19,4 @@ library
   default-language:    Haskell2010
   build-depends:       base >= 4.7 && < 5,
                        mtl == 2.2.1,
-                       transformers == 0.5.2.0
+                       transformers == 0.5.5.0

stack.yaml

@@ -39,7 +39,8 @@ packages:
 - .
 # Dependency packages to be pulled from upstream that are not in the resolver
 # (e.g., acme-missiles-0.3)
-extra-deps: []
+extra-deps:
+- transformers-0.5.5.0
 
 # Override default flag values for local packages and extra-deps
 flags: {}
@@ -63,4 +64,4 @@ extra-package-dbs: []
 # 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