Explicitly exit after running coverage.

If the tested code leaves threads or threadpools hanging, the jvm will
not automatically shut down. This causes `lein cloverage` to hang
indefinitely after producing output.

Fixes JacekLach/cloverage#24 and lshift/cloverage#26.
Don't overflow exit code (256 errors -> exit 0)
Add .lein-repl-history to gitignore.
Merge pull request #30 from JacekLach/exit-after-tests

Explicitly exit after running coverage.