Add support for Django2.2, drop support for Django2.1

.travis.yml

@@ -6,12 +6,12 @@ python:
  - 3.6
 
 env:
- - DJANGO_VERSION=1.11.18 PGDB=9.6
- - DJANGO_VERSION=1.11.18 PGDB=10
- - DJANGO_VERSION=2.0.10 PGDB=9.6
- - DJANGO_VERSION=2.0.10 PGDB=10
- - DJANGO_VERSION=2.1.5 PGDB=9.6
- - DJANGO_VERSION=2.1.5 PGDB=10
+ - DJANGO_VERSION=1.11.20 PGDB=9.6
+ - DJANGO_VERSION=1.11.20 PGDB=10
+ - DJANGO_VERSION=2.1.8 PGDB=9.6
+ - DJANGO_VERSION=2.1.8 PGDB=10
+ - DJANGO_VERSION=2.2.0 PGDB=9.6
+ - DJANGO_VERSION=2.2.0 PGDB=10
 
 before_install:
   - sudo -E service postgresql stop 9.2

README.md

@@ -22,7 +22,7 @@ Smartmin tries to stay in lock step with the latest Django versions. With each n
 will be released and we will save the major changes (possibly breaking backwards compatibility) on these versions.  This 
 includes updating to the latest version of Twitter Bootstrap.
 
-The latest version is the 2.1.* series which works against Django 1.11, 2.0 and 2.1.
+The latest version is the 2.2.* series which works against Django 1.11, 2.1 and 2.2.
 
 About
 =====

requirements/base.txt

@@ -1,5 +1,5 @@
 celery
-django>=1.11,<2.2
+django>=1.11,<3.0
 django_compressor
 pytz
 redis