Look for nrepl port in .nrepl-port as well.

Leiningen, Immutant, and soon mod-lang-clojure for Vert.x all write
the repl port to project-dir/.nrepl-port as a standard, well-known
location. This patch adds support for checking that file as well.

See https://github.com/technomancy/leiningen/issues/1296
Merge pull request #395 from tobias/nrepl-port

Look for nrepl port in .nrepl-port as well.