Crash if wrong version of extension dependencies

[jodal]

We handle missing extension dependencies nicely by simply disabling the extension:

$ mopidy
INFO     Disabled extension soundcloud: Dependency requests>=2.0.0 not found
INFO     Disabled extension beets: Dependency requests>=2.0.0 not found
INFO     Disabled extension gmusic: Dependency requests>=1.1.0 not found
INFO     Enabled extensions: spotify, mpd, stream, scrobbler, mpris, dirble, local
INFO     Disabled extensions: http, beets, nad, gmusic, subsonic, radio-de, arcam, soundcloud
...

Though, if an extension is installed, but has the wrong version, we crash and burn:

$ mopidy 
ERROR    (requests 1.2.3 (/usr/lib/python2.7/dist-packages), Requirement.parse('requests>=2.0.0'))
Traceback (most recent call last):
  File "/home/jodal/dev/mopidy/mopidy/__main__.py", line 78, in main
    if not ext.validate_extension(extension):
  File "/home/jodal/dev/mopidy/mopidy/ext.py", line 219, in validate_extension
    extension.entry_point.require()
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2027, in require
    working_set.resolve(self.dist.requires(self.extras),env,installer))
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 598, in resolve
    raise VersionConflict(dist,req) # XXX put more info here
VersionConflict: (requests 1.2.3 (/usr/lib/python2.7/dist-packages), Requirement.parse('requests>=2.0.0'))
Traceback (most recent call last):
  File "/home/jodal/dev/virtualenvs/mopidy/bin/mopidy", line 9, in <module>
    load_entry_point('Mopidy==0.18.0', 'console_scripts', 'mopidy')()
  File "/home/jodal/dev/mopidy/mopidy/__main__.py", line 78, in main
    if not ext.validate_extension(extension):
  File "/home/jodal/dev/mopidy/mopidy/ext.py", line 219, in validate_extension
    extension.entry_point.require()
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2027, in require
    working_set.resolve(self.dist.requires(self.extras),env,installer))
  File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 598, in resolve
    raise VersionConflict(dist,req) # XXX put more info here
pkg_resources.VersionConflict: (requests 1.2.3 (/usr/lib/python2.7/dist-packages), Requirement.parse('requests>=2.0.0'))

Having the wrong version should be handled just as nicely as not having the dependency.

[jodal]

With the fix, the following happens instead:

$ mopidy
INFO     Disabled extension soundcloud: requests>=2.0.0 required, but found requests 1.2.3 at /usr/lib/python2.7/dist-packages
INFO     Disabled extension beets: requests>=2.0.0 required, but found requests 1.2.3 at /usr/lib/python2.7/dist-packages
INFO     Enabled extensions: spotify, mpd, stream, scrobbler, mpris, dirble, local
INFO     Disabled extensions: http, beets, nad, gmusic, subsonic, radio-de, arcam, soundcloud