travis: modernize base build images when possible

We can use py2.7 on Bionic (newest image); it also supports 3.6+. This
is the majority of our required versions, so Bionic is now our default.

The other three Python versions we need to test for the life of Sopel
7.x require special treatment.

Travis only supports py3.3 on the Trusty build image, which is pretty
old by now but obviously still functional.

Our py3.4 & py3.5 builds remain on Xenial, as it's still the newest
build image to support those versions per Travis documentation.

.travis.yml

@@ -1,21 +1,23 @@
-dist: xenial  # required for newer (>= 3.7) Python versions
+dist: bionic  # most recent base image by default
 language: python
 matrix:
   include:
-    - name: "Python 2.7 on Xenial"
+    - name: "Python 2.7 on Bionic"
       python: 2.7
     - name: "Python 3.3 on Trusty"
       python: 3.3
-      dist: trusty  # xenial does not support py3.3
+      dist: trusty  # trusty is the last image to officially support py3.3
     - name: "Python 3.4 on Xenial"
       python: 3.4
+      dist: xenial  # xenial is the last image to officially support py3.4
     - name: "Python 3.5 on Xenial"
       python: 3.5
-    - name: "Python 3.6 on Xenial"
+      dist: xenial  # xenial is the last image to officially support py3.5
+    - name: "Python 3.6 on Bionic"
       python: 3.6
-    - name: "Python 3.7 on Xenial"
+    - name: "Python 3.7 on Bionic"
       python: 3.7
-    - name: "Python 3.8 on Xenial"
+    - name: "Python 3.8 on Bionic"
       python: 3.8
 git:
   submodules: false