Switch to using HEAD of paramiko

The maintainers of paramiko have now merged Scott Maxwell's pull
request to support Python 3 into the master branch. It should
appear in v1.14 of paramiko.

Update the requirements file to pull from paramiko@master for now
so that we get all the fixes that were included in the merge.

Later, the requirements will need to be modified to specify >=1.14
Allow empty query string in gsshcli.py

Querying for 'all open commits on project x' canbe performed without
actually providing a query string by using command options, e.g.

    ./gsshcli query --project=x --status=open gerrit.example.com

But this triggered an exception when forming the actual command
string.

Add a guard to condition the join() call on actually having a
value returned from the parser.