Travis: Fix caching directive
The `directories` key requires a list of directories (not a scalar) as
value. See also:
@@ -4,10 +4,10 @@ osx_image: xcode10.1
cache:
directories:
- $HOME/.m2
- $HOME/.cabal
- $HOME/elektra-cabal-sandbox
- $HOME/Library/Caches/Homebrew
+ - $HOME/.m2
+ - $HOME/.cabal
+ - $HOME/elektra-cabal-sandbox
+ - $HOME/Library/Caches/Homebrew
# don't cache our own libraries generated in the build
# so they always get rebuilt properly and to avoid init issues
