Warn when environment defaults are not strings

[DanBurton]

It seems fishy to me that one can set

:env {:port 5000}

And that this is loaded as the number 5000, but if one's environment variables override this with (for example) PORT=5001 lein run, then :port will be set to the string "5001". This can lead to subtle bugs.

It makes more sense to me to only allow string values in the environment map. Or at least, to log a warning when non-string environment default values are loaded.

[weavejester]

This issue is already covered by #36.