mediawiki: only provision ini files for igbinary and wmerrors on precise

Follows-up 16042c9c4b0.

We don't use these extensions on Trusty (and have no plans to). The Package
resources are already behind a $::lsbdistcodename check, so the ini files
should be, too.

Change-Id: I59046ec0b7fa926f5e7ffd04200cc4136af7a667