Update gitmodules

- externals/httplib2 now refers to our local mirror, with our patches
- externals/httplib2 and scripts/i18n now use the same URL format as
  https://github.com/wikimedia/mediawiki-extensions/blob/master/.gitmodules
  to make Gerrit automatically update them. Hopefully.
- externals/httplib2 and scripts/i18n have been updated to last master
  revision

Change-Id: If5d04cbcaa45bc4a5619c1424629b214d3933a25
Merge "Update gitmodules"