Add Python 3.7 and Django 1.11 to the build matrix

Fixes #1125.

Requires pinning mysqlclient<1.4 because of mysqlclient's failure to
interpolate named params into an SQL query (it converts strings to bytes
and afterwards complains that it cannot find b"first_name", e.g. in
test_raw_query_param_conversion:

  ...
  File "/home/travis/build/jazzband/django-debug-toolbar/.tox/mariadb/lib/python3.7/site-packages/MySQLdb/cursors.py", line 188, in execute
    query = query % args
  KeyError: b'first_name'

.travis.yml

@@ -13,6 +13,8 @@ matrix:
       env: TOXENV=py35-dj111
     - python: 3.6
       env: TOXENV=py36-dj111
+    - python: 3.7
+      env: TOXENV=py37-dj111
     - python: 3.4
       env: TOXENV=py34-dj20
     - python: 3.5

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py{27,34,35,36}-dj111
+    py{27,34,35,36,37}-dj111
     py{34,35,36,37}-dj20
     py{35,36,37}-dj21
     py{35,36,37}-djmaster
@@ -49,7 +49,7 @@ deps =
     coverage
     django_jinja
     html5lib
-    mysqlclient
+    mysqlclient<1.4
     selenium<4.0
     sqlparse
 setenv =