Bug 854121:  Regenerate sources.xml unconditionally rather than only when the underlying manifest changes.

It is quite common for the revisions underlying the build to change
without the manifest changing, for example, when "./repo sync" pulls a
few new gecko or gaia revisions.  This change makes us regenerate
sources.xml in those cases.
Merge pull request #76 from dbaron/master

Bug 854121:  Regenerate sources.xml unconditionally rather than only when the underlying manifest changes