Add install-cabal subcommand, fixes #3

.travis.sh

@@ -19,14 +19,13 @@ set -e
 ./ghcup -v set 8.2.2
 
 # install cabal-install
-wget https://www.haskell.org/cabal/release/cabal-install-2.2.0.0/cabal-install-2.2.0.0-x86_64-unknown-linux.tar.gz
-tar -xzf cabal-install-2.2.0.0-x86_64-unknown-linux.tar.gz
+./ghcup -v install-cabal
 
-export PATH="$HOME/.ghcup/bin:$PATH"
+export PATH="$HOME/.cabal/bin:$HOME/.ghcup/bin:$PATH"
 
 # install shellcheck
-./cabal update
-./cabal install ShellCheck
+cabal update
+cabal install ShellCheck
 
 # check our script for errors
-~/.cabal/bin/shellcheck ghcup
+shellcheck ghcup