Constrain cabal to non-ancient versions.

It has supported `extra-doc-files` for a long time, but apparently not always...

Author: leftaroundabout

dynamic-plot.cabal

@@ -27,7 +27,7 @@ Author:              Justus Sagemüller
 Maintainer:          (@) sagemuej $ smail.uni-koeln.de
 Homepage:            https://github.com/leftaroundabout/dynamic-plot
 Build-Type:          Simple
-Cabal-Version:       >=1.10
+Cabal-Version:       >=1.18
 Extra-Doc-Files:     images/examples/*.png
                      , images/examples/*.gif