copilot: Relax version constraint on optparse-applicative. Refs #488.

In order to keep Copilot effectively working in the current Haskell
ecosystem, we need to extend the versions of dependencies that Copilot
can be installed with. For example, optparse-applicative has seen a new
release 0.18.1.0, but copilot needs versions strictly lower than 0.18.

This commit relaxes the upper bounds on optparse-applicative so that it
can be installed with the 0.18 series.

Author: ivanperez-keera

copilot/copilot.cabal

@@ -48,7 +48,7 @@ library
       -fno-warn-orphans
     build-depends:
                        base                  >= 4.9  && < 5
-                     , optparse-applicative  >= 0.14 && < 0.18
+                     , optparse-applicative  >= 0.14 && < 0.19
                      , directory             >= 1.3  && < 1.4
                      , filepath              >= 1.4  && < 1.5