travis: Disable soup in "all" test

On Ubuntu 12.04, there seems to be a resource leak related to pthread keys
when initializing glib or related libraries more than once. With our repeated
initialization for libstrongswan tests, we hit the following error:

  Lib (gthread-posix.c): Unexpected error from C library during
  'pthread_key_create': Resource temporarily unavailable.

The problem is not reproducible on a newer Gnome stack, hence we disable the
glib based soup plugin until we have a more recent Ubuntu on Travis.