travis: Fix build by restricting docker version.
travis: Fix build by restricting docker version.

This commit forces the docker version used in travis to be 1.11.2-0~trusty.
It seems that some of the network unit tests are not working with the
latest version of docker.

Signed-off-by: Mark Ryan <f16215f7acd0bd7fdc465539215bd574c785b062@intel.com>

ciao-project/ciao,rbradford/ciao,mcastelino/ciao,mrkz/ciao,matthewrsj/ciao,mrkz/ciao,tpepper/ciao,01org/ciao,kaccardi/ciao,erick0z/ciao,markdryan/ciao,leoswaldo/ciao,matthewrsj/ciao,jorgevgut/ciao,obedmr/ciao,ciao-project/ciao,wcwxyz/ciao,erick0z/ciao,erick0z/ciao,mrkz/ciao,kaccardi/ciao,obedmr/ciao,matthewrsj/ciao,mcastelino/ciao,wcwxyz/ciao,leoswaldo/ciao,leoswaldo/ciao,jorgevgut/ciao,wcwxyz/ciao,rbradford/ciao,markdryan/ciao,mcastelino/ciao,obedmr/ciao,tpepper/ciao,01org/ciao,tpepper/ciao,jorgevgut/ciao