Fix Build.get_revision() for git repositories

With Jenkins, 1.5xx `Build.get_revision` is failing with the following error,

```
In [26]: b.get_revision()
---------------------------------------------------------------------------
AttributeError                            
Traceback (most recent call last)
<ipython-input-26-aac64a6bc23a> in <module>()
----> 1 b.get_revision()

/Users/sudharsh/venv/foobar/lib/python2.7/site-packages/jenkinsapi/build.pyc in get_revision(self)
     45     def get_revision(self):
     46         vcs = self._data['changeSet']['kind'] or 'git'
---> 47         return getattr(self, '_get_%s_rev' % vcs, lambda: None)()
     48
     49     def _get_svn_rev(self):

/Users/sudharsh/venv/foobar/lib/python2.7/site-packages/jenkinsapi/build.pyc in _get_git_rev(self)
     55     def _get_git_rev(self):
     56         for item in self._data['actions']:
---> 57             branch = item.get('buildsByBranchName')
     58             head = branch and branch.get('origin/HEAD')
     59             if head:

AttributeError: 'NoneType' object has no attribute 'get'
-----
```

Also, the `branch` was `origin/master` instead of `origin/HEAD` as `_get_git_rev` expects. 

The attached patch removes this dependancy and will work on any remote.