Description of problem:
Bodhi is failing on systems without "koji" package.

Version-Release number of selected component (if applicable):
bodhi-client-2.10.1-3.fc27.noarch

How reproducible:
Always

Steps to Reproduce:
1. Have system without koji package
2. Try to download some update via bodhi-client

Actual results:
$ bodhi updates download --updateid FEDORA-2017-43fcb10151
Downloading packages from python-sync2jira-1.3-1.fc27
Traceback (most recent call last):
  File "/usr/bin/bodhi", line 11, in <module>
    load_entry_point('bodhi-client==2.10.1', 'console_scripts', 'bodhi')()
  File "/usr/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/bodhi/client/__init__.py", line 111, in wrapper
    method(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/bodhi/client/__init__.py", line 481, in download
    ret = subprocess.call(args)
  File "/usr/lib64/python2.7/subprocess.py", line 168, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib64/python2.7/subprocess.py", line 390, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.7/subprocess.py", line 1024, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory


Expected results:
bodhi-client should depend on koji package.