Enable bionic build replacing xenail

Enable bionic 18.04 replacing xenial 16.04.  Curl
https://oligarchy.co.uk/xapian/ is failing to validate the SSL cert
with the CA on xenial. See if this fixes it.

Python 3.4 is not supported on bionic is obsolete so remove testing.
It was retained because 3.4 was EPEL version for centos 7. With centos
demise, removed it from 'python:' settings.

Test latest 2.x release, earliest 3.x release supported by bionic
(3.6), last two production 3.x releases and nightly to cut down on
cost of testing. Also run tests in 3.x series in reverse order so 3.9
runs first. Hence we fail early as newest pythons seem to have more
issues with the code.

Author: rouilj

.travis.yml

@@ -7,16 +7,6 @@ language: python
 
 cache: pip
 
-python:
-  - 2.7
-  - 3.9-dev
-  - 3.8
-  - 3.7
-  - 3.6
-  - 3.4
-  - nightly
-
-
 #I would like to build and test the maint-1.6 and trunk/default
 #but we need different environments for these:
 #  maint-1.6 only python 2, install only psycopg2 version with support for
@@ -27,29 +17,18 @@ branches:
 #    - maint-1.6
 
 dist:
- - xenial
-
-# Commented out stanza for bionic 18.04. Currently testing on
-# xenial 16.04.
-# Consider move to this after 2.1.0 release. Python 3.4
-# is not supported on bionic and 3.4 is obsolete. Was retained
-# because 3.4 was EPEL version for centos 7. With centos demise,
-# remove it from 'python:' settings and test earliest still supported
-# release, last two production releases and nightly to cut down on cost
-# of testing.
-# dist:
-#   - bionic
-#
-# python:
-#   - 2.7
-#   - 3.6
-#   - 3.8
-#   - 3.9-dev
-#   - nightly
-#
-# services:
-#   - mysql
-#   - postgresql
+  - bionic
+
+python:
+  - 2.7
+  - 3.9-dev
+  - 3.8
+  - 3.6
+  - nightly
+
+services:
+  - mysql
+  - postgresql
 
 jobs:
     allow_failures:  # nightly not ready for prime time yet.