Update Travis CI Matrix

Updates are:

 - Remove Django 1.11 from allowed failures

 - Add Python 3.7 tests

 - Add Django 2.1 tests

 - Add database services

 - Exclude Python 3.4 tests with MySQL database
   (because new versions of mysqlclient don't support Python 3.4)

.travis.yml

@@ -1,32 +1,50 @@
 language: python
+dist: xenial
+sudo: false
 python:
 - '2.7'
 - '3.4'
 - '3.5'
 - '3.6'
+- '3.7'
 env:
 - DJANGO="Django>=1.11.0,<2.0"    DB=postgresql DB_NAME=travis_ci_test
 - DJANGO="Django>=2.0,<2.1.0"     DB=postgresql DB_NAME=travis_ci_test
+- DJANGO="Django>=2.1,<2.2.0"     DB=postgresql DB_NAME=travis_ci_test
 - DJANGO="Django>=1.11.0,<2.0"    DB=sqlite3 DB_NAME=db.sqlite3
 - DJANGO="Django>=2.0,<2.1.0"     DB=sqlite3 DB_NAME=db.sqlite3
+- DJANGO="Django>=2.1,<2.2.0"     DB=sqlite3 DB_NAME=db.sqlite3
 - DJANGO="Django>=1.11.0,<2.0"    DB=mysql DB_NAME=mysql_db
 - DJANGO="Django>=2.0,<2.1.0"     DB=mysql DB_NAME=mysql_db
+- DJANGO="Django>=2.1,<2.2.0"     DB=mysql DB_NAME=mysql_db
 matrix:
-  allow_failures:
-  - python: '2.7'
-    env: DJANGO="Django>=1.11.0,<2.0"
-  - python: '3.4'
-    env: DJANGO="Django>=1.11.0,<2.0"
-  - python: '3.5'
-    env: DJANGO="Django>=1.11.0,<2.0"
   exclude:
   - python: '2.7'
     env: DJANGO="Django>=2.0,<2.1.0"     DB=postgresql DB_NAME=travis_ci_test
+  - python: '2.7'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=postgresql DB_NAME=travis_ci_test
   - python: '2.7'
     env: DJANGO="Django>=2.0,<2.1.0"     DB=sqlite3 DB_NAME=db.sqlite3
+  - python: '2.7'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=sqlite3 DB_NAME=db.sqlite3
   - python: '2.7'
     env: DJANGO="Django>=2.0,<2.1.0"     DB=mysql DB_NAME=mysql_db
+  - python: '2.7'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=mysql DB_NAME=mysql_db
+  - python: '3.4'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=postgresql DB_NAME=travis_ci_test
+  - python: '3.4'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=sqlite3 DB_NAME=db.sqlite3
+  - python: '3.4'
+    env: DJANGO="Django>=1.11.0,<2.0"    DB=mysql DB_NAME=mysql_db
+  - python: '3.4'
+    env: DJANGO="Django>=2.0,<2.1.0"     DB=mysql DB_NAME=mysql_db
+  - python: '3.4'
+    env: DJANGO="Django>=2.1,<2.2.0"     DB=mysql DB_NAME=mysql_db
   fast_finish: true
+services:
+  - mysql
+  - postgresql
 install:
 - travis_retry pip install -q $DJANGO
 - travis_retry pip install -r requirements.txt