Add-on shutdown: Need to flush the string bundle service cache

[Yvon TANGUY]

When the add-on is upgraded, string bundle files may change.
If the cache is not flushed, when a bootstrap's add-on is upgraded or downgraded, and the string bundle have changes,
this add-on may not work correctly.

See:
- http://maglione-k.users.sourceforge.net/bootstrapped.xhtml (String bundles)
- https://developer.mozilla.org/en-US/docs/Mozilla/XPCOM/Reference/Interface/nsIStringBundleService#flushBundles%28%29

[Yvon TANGUY (vono22)]

do it also for add-on upgrade.
For 1.8.0: only if old version < 1.7.4