add py37 / Django 2.1 to travis, restructure matrix

.travis.yml

@@ -4,16 +4,6 @@ language: python
 
 os: linux
 
-dist: trusty
-
-python:
-    - "2.7"
-    - "pypy"
-    - "3.4"
-    - "3.5"
-    - "3.6"
-    - "3.6-dev"
-
 env:
     global:
       - SECRET_KEY=justfortravis
@@ -22,24 +12,24 @@ env:
       - TEST_K="not ddns_client"
       - TEST_OPTS=""
       - COVERAGE=""
-    matrix:
-      - DJANGO=1.11
 
 matrix:
-  exclude:
-    - python: "2.7"
-      env: DJANGO=1.11
-    - python: "3.6"
-      env: DJANGO=1.11
-    - python: "3.6-dev"
-      env: DJANGO=1.11
   include:
     - python: "2.7"
+      dist: trusty
       env: DJANGO=1.11 COVERAGE="coverage run -m" TEST_K="not ddns_client" TEST_OPTS="--pep8"
-    - python: "3.6"
+    - python: "3.4"
+      dist: trusty
       env: DJANGO=1.11
-    - python: "3.6-dev"
-      env: DJANGO=2.0
+    - python: "3.5"
+      dist: trusty
+      env: DJANGO=1.11
+    - python: "3.6"
+      dist: trusty
+      env: DJANGO=1.11
+    - python: "3.7"
+      dist: xenial
+      env: DJANGO=2.1
 
 install:
     - ./scripts/travis/install.sh