travis: Remove 'sudo' configuration.

Since TravisCI migrated jobs from containers to VMs, 'sudo' is always
available. Setting 'sudo: false' is misleading because it makes no
effect.

https://docs.travis-ci.com/user/reference/trusty/#container-based-infrastructure

 "Container-based infrastructure is currently being deprecated.
  Please remove any sudo: false keys in your .travis.yml file to use
  the default fully-virtualized Linux infrastructure instead."

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

.travis.yml

@@ -21,8 +21,6 @@ before_install: ./.travis/${TRAVIS_OS_NAME}-prepare.sh
 
 before_script: export PATH=$PATH:$HOME/bin
 
-sudo: false
-
 env:
   - OPTS="--disable-ssl"
   - TESTSUITE=1 KERNEL=3.16.39