switch to xenial and test Python 3.7/3.8

Author: videlec

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: xenial
 sudo: required
 language: python
 notifications:
@@ -10,6 +10,8 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
 env:
   - PARI_VERSION=pari-2.9.4
     URLDIR=OLD/2.9