travis: replace recently missing links for libyaml

Rerunning previously passing integration test shows that the
multilibbed libyaml libraries are no longer available from
/usr/lib alongside other libs - maybe an OS upgrade, or an
upstream packaging error?
* .travis.yml (install): Run a shell find command to link the
multilib libyaml libs back into /usr/lib.
If this command starts failing in future then it should mean
libyaml has moved back to the proper place causing our manual
relink to start failing, so this workaround can then be
removed.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>