fix thread/file handle leaks in API tests (Fixes #151)

Bump unfiltered version to 0.8.1
Explicitly close the dispatch request handler
Bind local port only for tests
fix Travis thread exhaustion

Reduce max parallel threads by waiting a bit after (asynchronously)
shutting down each unfiltered/netty server instance
wait on the server's thread pool's shutdown future
Merge pull request #261 from mossprescott/151-api-test-leak

API tests: fix file handle leak in SBT, thread exhaustion in Travis