'signac config verify' is broken

[csadorf]

Description

The signac config verify CLI command is currently broken, as was discovered during the implementation of #300 .

To reproduce

Execute signac config verify on the command line.

~/local/signac $ signac --debug config verify
DEBUG:signac.common.config:Reading config file '/Users/csadorf/.signacrc'.
DEBUG:signac.common.validate:Install pymongo to validate database configurations!
# ... omitting many repeats of the above message
DEBUG:signac.common.config:Config file '/Users/csadorf/.signacrc' may contain invalid values.
DEBUG:signac.common.config:Reached filesystem root.
Error: validate() got an unexpected keyword argument 'skip_missing'
Traceback (most recent call last):
  File "/Users/csadorf/miniconda3/envs/signac-dev/bin/signac", line 11, in <module>
    load_entry_point('signac', 'console_scripts', 'signac')()
  File "/Users/csadorf/local/signac/signac/__main__.py", line 1835, in main
    args.func(args)
  File "/Users/csadorf/local/signac/signac/__main__.py", line 809, in main_config_verify
    verify_config(cfg)
  File "/Users/csadorf/local/signac/signac/__main__.py", line 721, in verify_config
    verification = cfg.verify(
  File "/Users/csadorf/local/signac/signac/common/config.py", line 133, in verify
    return super(Config, self).validate(validator, *args, **kwargs)
TypeError: validate() got an unexpected keyword argument 'skip_missing'

Error output

If possible, copy any terminal outputs or attach screenshots that provide additional information on the problem.

System configuration

macOS-10.11.6-x86_64-i386-64bit
3.8.1 | packaged by conda-forge | (default, Jan 29 2020, 15:06:10)
[Clang 9.0.1 ]
1.4.0