ci: Use "dist: xenial" in Travis

To test in python >= 3.7 it is required to have a newer Ubuntu version.

Xenial has a newer version of PyPy.  Update Travis template to use
'pypy3.5'.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

.travis.yml

@@ -1,3 +1,4 @@
+dist: xenial
 language: python
 sudo: false
 cache: pip
@@ -40,10 +41,10 @@ matrix:
     - python: '3.7'
       env:
         - TOXENV=3.7-nocov
-    - python: 'pypy-5.4'
+    - python: 'pypy3.5'
       env:
         - TOXENV=pypy-cover,report,coveralls,codecov
-    - python: 'pypy-5.4'
+    - python: 'pypy3.5'
       env:
         - TOXENV=pypy-nocov
 before_install:

ci/templates/.travis.yml

@@ -1,3 +1,4 @@
+dist: xenial
 language: python
 sudo: false
 cache: pip
@@ -11,7 +12,7 @@ env:
 matrix:
   include:
 {%- for env, config in tox_environments|dictsort %}{{ '' }}
-    - python: '{{ '{0[0]}-5.4'.format(env.split('-')) if env.startswith('pypy') else env.split('-')[0] }}'
+    - python: '{{ '{0[0]}3.5'.format(env.split('-')) if env.startswith('pypy') else env.split('-')[0] }}'
       env:
         - TOXENV={{ env }}{% if "true" in config.cover %},report,coveralls,codecov{% endif -%}
 {% endfor %}