Use bionic image in Travis tests

Author: blag

.travis.yml

@@ -1,6 +1,6 @@
 # Travis infra requires pinning dist:precise, at least as of 2017-09-01
 # detail: https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch
-dist: precise
+dist: bionic
 language: python
 install:
   - pip install -U setuptools && pip install -U tox codecov tox-travis
@@ -47,28 +47,16 @@ matrix:
     - python: 3.6
       env: TOXENV=py36-compatibility
     # CPython 3.7
-    # xenial + sudo are currently needed to get 3.7
-    # https://github.com/travis-ci/travis-ci/issues/9815
     - python: 3.7
       env: TOXENV=py37-base
-      dist: xenial
-      sudo: true
     - python: 3.7
       env: TOXENV=py37-cryptography-only
-      dist: xenial
-      sudo: true
     - python: 3.7
       env: TOXENV=py37-pycryptodome-norsa
-      dist: xenial
-      sudo: true
     - python: 3.7
       env: TOXENV=py37-pycrypto-norsa
-      dist: xenial
-      sudo: true
     - python: 3.7
       env: TOXENV=py37-compatibility
-      dist: xenial
-      sudo: true
     # PyPy 3.5 (5.10.1?)
     - python: pypy3.5
       env: TOXENV=pypy-base