travis: the sudo tag is now deprecated (#11933)

* Travis CI: The sudo tag is now deprecated in Travis

__sudo: required__ no longer is.

* Remove obsolete comment

__travis_wait__ can help with long-running processes.
* https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

.travis.yml

@@ -19,39 +19,32 @@ jobs:
   include:
     - stage: 'style checks'
       python: '2.7'
-      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=flake8
     - stage: 'unit tests + documentation'
       python: '2.6'
       dist: trusty
-      sudo: required
       os: linux
       language: python
       env: [ TEST_SUITE=unit, COVERAGE=true ]
     - python: '2.7'
-      sudo: required
       os: linux
       language: python
       env: [ TEST_SUITE=unit, COVERAGE=true ]
     - python: '3.5'
-      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=unit
     - python: '3.6'
-      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=unit
     - python: '3.7'
-      sudo: required
       os: linux
       language: python
       env: [ TEST_SUITE=unit, COVERAGE=true ]
     - python: '3.7'
-      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=doc
@@ -101,7 +94,6 @@ jobs:
       env: [ TEST_SUITE=build, 'SPEC=mpich' ]
     - python: '3.6'
       stage: 'docker build'
-      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=docker
@@ -119,8 +111,6 @@ stages:
 #=============================================================================
 # Environment
 #=============================================================================
-# Use new Travis infrastructure (Docker can't sudo yet)
-sudo: false
 
 # Docs need graphviz to build
 addons: