Don't test with GHC 7.4 to 7.8

.travis.yml

 sudo: required
 env:
-- GHCVER=7.4
-- GHCVER=7.6
-- GHCVER=7.8
 - GHCVER=7.10
 - GHCVER=8.0
 - GHCVER=8.2

filepath.cabal

@@ -12,7 +12,7 @@ homepage:       https://github.com/haskell/filepath#readme
 category:       System
 build-type:     Simple
 synopsis:       Library for manipulating FilePaths in a cross platform way.
-tested-with:    GHC==8.6.3, GHC==8.4.4, GHC==8.2.2, GHC==8.0.2, GHC==7.10.3, GHC==7.8.4, GHC==7.6.3, GHC==7.4.2
+tested-with:    GHC==8.6.3, GHC==8.4.4, GHC==8.2.2, GHC==8.0.2, GHC==7.10.3
 description:
     This package provides functionality for manipulating @FilePath@ values, and is shipped with both <https://www.haskell.org/ghc/ GHC> and the <https://www.haskell.org/platform/ Haskell Platform>. It provides three modules:
     .