ci: Try to reduce disk space required

Travis fails because the disk is full.

.travis.yml

@@ -1,18 +1,24 @@
-# Use new container infrastructure to enable caching
 sudo: false
 
-language: haskell
+language: generic
 
 # Caching so the next build will be fast too.
 cache:
   directories:
-  - $HOME/.stack
+  - $HOME/.stack/
+  - $HOME/.local/bin/
+  - .stack-work/
 
 before_install:
+  - set -e
+
+  # stack
   - mkdir -p ~/.local/bin
   - export PATH=~/.local/bin:$PATH
-  - travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'
-
+  - if [[ ! -x ~/.local/bin/stack ]]; then travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'; chmod a+x ~/.local/bin/stack; fi
+  - stack --version
+  - df -h
+
 matrix:
   include:
   - env: ARGS=""