Travis: upgrade to Ubuntu 20.04 (#203)

* Travis: upgrade to Ubuntu 20.04

* Travis: Explicitly list all jobs

This is in preparation for adding arm64.

Author: ulfjack

.travis.yml

@@ -1,9 +1,13 @@
-os:
-  - linux
-  - osx
-  - windows
+jobs:
+  include:
+  - os: linux
+    arch: amd64
+  - os:osx
+    arch: amd64
+  - os:windows
+    arch: amd64
 
-dist: bionic
+dist: focal
 osx_image: xcode12
 
 cache: