Travis CI: 'sudo' is no longer required by Travis

[Travis are now recommending removing __sudo__](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration)

.travis.yml

@@ -1,19 +1,15 @@
 language: python
 
-sudo: false
-
 matrix:
     include:
         - python: 2.7
           env: TOXENV=cover
         - python: 3.7
           env: TOXENV=cover3
           dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
-          sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
         - python: 3.7
           env: TOXENV=docs
           dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
-          sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
         - python: 2.7
           env:
             - TOXENV=py27
@@ -35,7 +31,6 @@ matrix:
             - TOXENV=py37
             - END_TO_END=1
           dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
-          sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
 
 install:
     - travis_retry pip install virtualenv tox pexpect