Update test git-server dependency to 1.5
Update to use surefire plugin 2.18
Use null as privatekey in credentials test if private key file doesn't exist

The credentials test needs a better data file format than the current
simple minded CSV file which contains URL, username, and private key
file.  Some valid credential test cases don't need a private key, and
other valid credential test cases don't need either a username or a
private key.

Until the replacement for the CSV file is available, this temporary
technique will allow a few more cases to be tested, without major
changes to the credentials test code.