Revert "Work around haskell/cabal#4624"

This reverts commit ef085d1.

Follow @hvr's advice in
haskell/cabal#4624 (comment)

.travis.yml

@@ -13,27 +13,27 @@ before_cache:
 
 matrix:
   include:
-    - env: CABALVER=2.0 GHCVER=7.0.4
+    - env: CABALVER=1.24 GHCVER=7.0.4
       compiler: ": #GHC 7.0.4"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.0.4], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=7.2.2
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.0.4], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.2.2
       compiler: ": #GHC 7.2.2"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.2.2], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=7.4.2
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.2.2], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.4.2
       compiler: ": #GHC 7.4.2"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.4.2], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=7.6.3
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.4.2], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.6.3
       compiler: ": #GHC 7.6.3"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.6.3], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=7.8.4
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.6.3], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.8.4
       compiler: ": #GHC 7.8.4"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.8.4], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=7.10.3
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.8.4], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=7.10.3
       compiler: ": #GHC 7.10.3"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
-    - env: CABALVER=2.0 GHCVER=8.0.2
+      addons: {apt: {packages: [cabal-install-1.24,ghc-7.10.3], sources: [hvr-ghc]}}
+    - env: CABALVER=1.24 GHCVER=8.0.2
       compiler: ": #GHC 8.0.2"
-      addons: {apt: {packages: [cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [cabal-install-1.24,ghc-8.0.2], sources: [hvr-ghc]}}
     - env: CABALVER=2.0 GHCVER=8.2.1
       compiler: ": #GHC 8.2.1"
       addons: {apt: {packages: [cabal-install-2.0,ghc-8.2.1], sources: [hvr-ghc]}}
@@ -42,8 +42,8 @@ matrix:
       addons: {apt: {packages: [cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
   allow_failures:
-    - env: CABALVER=2.0 GHCVER=7.0.4
-    - env: CABALVER=2.0 GHCVER=7.2.2
+    - env: CABALVER=1.24 GHCVER=7.0.4
+    - env: CABALVER=1.24 GHCVER=7.2.2
     - env: CABALVER=head GHCVER=head
 
 before_install: