Fixes error message.
Be proactive about Node unstable, but do not fail.

At the moment we fail the build if it does not pass in the latest Node
unstable.  This is undesirable since we only officially support stable
versions.  However it's nice to be proactive about this version:

http://weblog.bocoup.com/proactive-nodejs-development/
Updated README to show the correct Travis-CI badge.
Merge pull request #139 from nodegit/fix-tests

Fix and improve testing.