Update .travis.yml

Update .travis.yml

The travis build is often blocked by the following error

  $ sudo apt-get install -y build-essential
  E: Could not get lock /var/lib/dpkg/lock - open (11: Resource
temporarily unavailable)

Feedback from travis is the dpkg lock file might be locked by an
unattended `apt-get` call that's running in the background on a daily
basis and travis suggests adding the corresponding snippet before
apt-get call in .travis.yml

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>