travis: require sudo in an attempt to avoid build timeouts (#9327)

- Many container builds are timing out frequently during Spack tests in
  Travis CI.

- Travis recommends to try `sudo: required` to see whether this is an
  infrastructure issue or something else.

- added `sudo: required` to all Linux builds.

- added --verbose to `spack test` invocation so that we can see more
  easily what tests it's timing out on.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

.travis.yml

@@ -23,37 +23,44 @@ 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'
+      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=unit
     - python: '2.7'
+      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=unit
     - python: '3.4'
+      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=unit
     - 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
       dist: xenial
-      sudo: true
       language: python
       env: TEST_SUITE=unit
     - python: '3.6'
+      sudo: required
       os: linux
       language: python
       env: TEST_SUITE=doc