Conflict with latest version of pip

[jtschichold]

MineMeld Engine doesn't start with latest version of pip:

2018-04-17T07:47:58 (5408)launcher.main INFO: mm-run.py arguments: Namespace(config='/opt/minemeld/local/config', multiprocessing=0, nodes_per_chassis=15.0, verbose=False)
Traceback (most recent call last):
  File "/opt/minemeld/engine/current/bin/mm-run", line 11, in <module>
    sys.exit(main())
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/run/launcher.py", line 218, in main
    config = minemeld.run.config.load_config(args.config)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/run/config.py", line 567, in load_config
    return _load_config_from_dir(config_path)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/run/config.py", line 382, in _load_config_from_dir
    ccvalid, cconfig = _load_and_validate_config_from_file(ccpath)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/run/config.py", line 281, in _load_and_validate_config_from_file
    valid = resolve_prototypes(config)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/run/config.py", line 477, in resolve_prototypes
    prototypes_entrypoints = minemeld.loader.map(minemeld.loader.MM_PROTOTYPES_ENTRYPOINT)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/loader.py", line 105, in map
    _initialize_entry_point_group(entrypoint_group)
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/loader.py", line 55, in _initialize_entry_point_group
    installed = _installed_versions()
  File "/opt/minemeld/engine/0.9.44/local/lib/python2.7/site-packages/minemeld/loader.py", line 29, in _installed_versions
    installed_dists = pip.get_installed_distributions()
AttributeError: 'module' object has no attribute 'get_installed_distributions'