Relax upper bound on `time` to allow time-1.7

...after having convincing myself that the changes in time-1.7 are confined
to parts not used by `unix`

c.f. http://hdiff.luite.com/cgit/time/diff?id=1.7&id2=1.6.0.1

fixes #80

unix.cabal

@@ -64,7 +64,7 @@ library
     build-depends:
         base        >= 4.5     && < 4.11,
         bytestring  >= 0.9.2   && < 0.11,
-        time        >= 1.2     && < 1.7
+        time        >= 1.2     && < 1.8
 
     exposed-modules:
         System.Posix