awkward phrasing for message about source not supporting changelogs

The current message for a source that does not support changelogs is

  "This change is not coming from a source that supports changelogs."

This form of the verb "to come" is slightly awkward in this context (at least, in American English, my native language). This present continuous "is coming" is something one might say if something is happening currently or continuously. I suggest the following message:

  "This update does not come from a source that supports changelogs."

This also changes "This change" to "This update", because there may be more than one change in each update.

I have attached a patch which changes the message in the po/update-manager.pot file and where it occurs in the UpdateManager/Core/MyCache.py file.