Remove support for GHC 7.10

`hnix` has a `base >= 4.9` requirement which prevents building with
older version of GHC

dhall-nix/.travis.yml

@@ -13,9 +13,6 @@ before_cache:
 
 matrix:
   include:
-    - env: CABALVER=1.22 GHCVER=7.10.2
-      compiler: ": #GHC 7.10.2"
-      addons: {apt: {packages: [cabal-install-1.22,ghc-7.10.2], sources: [hvr-ghc]}}
     - env: CABALVER=1.24 GHCVER=8.0.1
       compiler: ": #GHC 8.0.1"
       addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.1], sources: [hvr-ghc]}}

dhall-nix/dhall-nix.cabal

@@ -2,7 +2,7 @@ Name: dhall-nix
 Version: 1.0.0
 Cabal-Version: >=1.8.0.2
 Build-Type: Simple
-Tested-With: GHC == 7.10.2, GHC == 8.0.1
+Tested-With: GHC == 8.0.1
 License: BSD3
 License-File: LICENSE
 Copyright: 2017 Gabriel Gonzalez