Add upper bound to ‘path-io’ dependency

There will be breaking changes in version 1.0.0 that affect some
functions that Stack uses, so it's better to have this upper bound for
now.

stack.cabal

@@ -175,7 +175,7 @@ library
                    , mtl >= 2.1.3.1
                    , optparse-applicative >= 0.11 && < 0.13
                    , path >= 0.5.1
-                   , path-io >= 0.3.1
+                   , path-io >= 0.3.1 && < 1.0.0
                    , persistent >= 2.1.2
                    , persistent-sqlite >= 2.1.4
                    , persistent-template >= 2.1.1