update GHC versions used on travis

I saw that (nightly) travis builds are failing because we now only support 8.4 or higher to boot a GHC HEAD build. I don't know how to achieve this for the OS X section though.

.travis.yml

@@ -3,56 +3,56 @@ matrix:
     include:
         - os: linux
           env: MODE="selftest"
-          compiler: "GHC 8.2.2"
+          compiler: "GHC 8.4.3"
           addons:
               apt:
                   packages:
-                      - ghc-8.2.2
-                      - cabal-install-2.2
+                      - ghc-8.4.3
+                      - cabal-install-2.4
                       - zlib1g-dev
                   sources: hvr-ghc
 
           before_install:
-              - PATH="/opt/ghc/8.2.2/bin:$PATH"
-              - PATH="/opt/cabal/2.2/bin:$PATH"
+              - PATH="/opt/ghc/8.4.3/bin:$PATH"
+              - PATH="/opt/cabal/2.4/bin:$PATH"
 
           script:
               # Run internal Hadrian tests, after boot and configure.
               - hadrian/build.sh -c selftest
 
         - os: linux
           env: MODE="--flavour=quickest"
-          compiler: "GHC 8.2.2"
+          compiler: "GHC 8.4.3"
           addons:
               apt:
                   packages:
-                      - ghc-8.2.2
-                      - cabal-install-2.2
+                      - ghc-8.4.3
+                      - cabal-install-2.4
                       - zlib1g-dev
                   sources: hvr-ghc
 
           before_install:
-              - PATH="/opt/ghc/8.2.2/bin:$PATH"
-              - PATH="/opt/cabal/2.2/bin:$PATH"
+              - PATH="/opt/ghc/8.4.3/bin:$PATH"
+              - PATH="/opt/cabal/2.4/bin:$PATH"
 
           script:
               # Build GHC, letting hadrian boot & configure the ghc source tree
               - hadrian/build.sh -c -j $MODE --no-progress --progress-colour=never --profile=-
 
         - os: linux
           env: MODE="--flavour=quickest --integer-simple"
-          compiler: "GHC 8.4.1"
+          compiler: "GHC 8.6.1"
           addons:
               apt:
                   packages:
-                      - ghc-8.4.1
-                      - cabal-install-2.2
+                      - ghc-8.6.1
+                      - cabal-install-2.4
                       - zlib1g-dev
                   sources: hvr-ghc
 
           before_install:
-              - PATH="/opt/ghc/8.4.1/bin:$PATH"
-              - PATH="/opt/cabal/2.2/bin:$PATH"
+              - PATH="/opt/ghc/8.6.1/bin:$PATH"
+              - PATH="/opt/cabal/2.4/bin:$PATH"
 
           script:
               # boot, configure and build GHC