rewrite GridSelect.stringToRatio to use randomR (fixes #572)

Due to differences between random-1.1 and random-1.2, on newer systems
stringToRatio returns numbers outside [0, 1] range, which breaks
colorRangeFromClassName colorizers.

This commit fixes the issue by using randomR to directly generate the random number.

Also this fixes the compilation warning (genRange and next are deprecated in random-1.2).

tests/GridSelect.hs

@@ -0,0 +1,15 @@
+module GridSelect where
+
+import Test.Hspec
+import Test.Hspec.QuickCheck
+
+import XMonad.Actions.GridSelect
+
+spec :: Spec
+spec = do
+  prop "prop_stringToRatio_valuesInRange"  prop_stringToRatio_valuesInRange
+
+prop_stringToRatio_valuesInRange :: String -> Bool
+prop_stringToRatio_valuesInRange s =
+  let r = stringToRatio s
+  in r >= 0 && r <= 1