fix bug around assuming "master" to be default branch, see #43

To reproduce bug, use a repo with a default branch != master, yet specify "master" as version explicitly, e.g.

- git: {local-name: ant, uri: 'git://github.com/apache/ant.git', version: 'master'}

Result after rosws call: local client checked out with "trunk" branch, and no error message that no master exists. Should instead give that error (or be on master branch)