travis: py37 should no longer require sudo

(once upon a time, sudo:true was necessary to get onto a GCE instance, which
has xenail which has py37)

.travis.yml

@@ -24,9 +24,6 @@ matrix:
     - python: 3.6
       script: tox -e py36-coverage
     - python: 3.7
-      # we don't actually need sudo, but that kicks us onto GCE, which lets
-      # us get xenial
-      sudo: true
       dist: xenial
       script: tox -e py37-coverage
     - python: nightly