Raised lower bound on base to 4.5. Fixes #18

This technically has no effect because `turtle` depends on `exceptions`,
which already has this lower bound, but it's probably a good idea to put
this lower bound anyway since I haven't verified that versions of `base`
less than 4.5 even work.

turtle.cabal

@@ -46,7 +46,7 @@ Source-Repository head
 Library
     HS-Source-Dirs: src
     Build-Depends:
-        base            >= 4       && < 5  ,
+        base            >= 4.5     && < 5  ,
         async           >= 2.0.0.0 && < 2.1,
         clock           >= 0.4.1.2 && < 0.5,
         directory                     < 1.3,