travis: try ubuntu 18.04 again

because pypy3 + httpx is not happy with a ssl library lacking ALPN support
https://travis-ci.org/github/lilydjwg/nvchecker/jobs/711675911

.travis.yml

@@ -1,5 +1,5 @@
 os: linux
-dist: xenial
+dist: bionic
 language: python
 cache: pip
 python:
@@ -8,7 +8,7 @@ python:
   - "3.7"
   - "3.8"
   - "nightly"
-  - "pypy3.6-7.1.1"
+  - "pypy3.6-7.3.1"
 install: pip install -U $DEPS pytest pytest-asyncio pytest-httpbin flaky structlog
 script: pytest
 env:
@@ -25,13 +25,7 @@ jobs:
   fast_finish: true
   allow_failures:
     # doesn't work well, see https://travis-ci.org/lilydjwg/nvchecker/jobs/376326582
-    - python: pypy3.6-7.1.1
+    - python: pypy3.6-7.3.1
       env: DEPS=aiohttp
     # it works now, but may be broken in the future again
     - python: nightly
-
-addons:
-  apt:
-    packages:
-    # This is needed for pycurl
-    - libgnutls-dev