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.
:main causes :aot since lein 2.0. That's bad.
Merge pull request #28 from JacekLach/aot-fix

Cloverage leaking its dependencies onto the tested classpath causes crash
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.