We can shorten our Travis cache paths

[petertseng]

We discovered in #406 -

Right now our Travis cache paths look something like

/home/travis/.foldercache/home/travis/build/exercism/xhaskell/exercises/anagram/list-string/.stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/Anagram.dyn_hi

/home/travis/.foldercache/home/travis/build/exercism/xhaskell/exercises/anagram/list-string/.stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/Anagram.dyn_o

There are many unnecessary path components in there!! It can just be /home/travis/.foldercache/anagram/list-string/.stack-work/...

So we can instead of using .foldercache/${exercise}, use .foldercache/$(basename $exercise).