Update Travis test matrix

- remove Python 3.6 for now as setuptools is broken there, see
  pypa/setuptools#866
- add Django 1.11 pre release, this will fail at least until fix for
  python-social-auth/social-app-django#24 is
  released

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -17,6 +17,7 @@ env:
     - CI_PIP_DEPS="Django>=1.10,<1.11" CI_DATABASE=mysql
     - CI_PIP_DEPS="Django>=1.10,<1.11" CI_DATABASE=postgresql
     - CI_PIP_DEPS="Django>=1.10,<1.11" CI_DATABASE=postgresql CI_MODE=selenium
+    - CI_PIP_DEPS="--pre Django"
     - CI_PIP_DEPS="sphinxcontrib-httpdomain docutils!=0.13.1" CI_MODE=docs
   global:
     - DJANGO_SETTINGS_MODULE=weblate.settings_test
@@ -45,11 +46,9 @@ matrix:
   include:
     - python: "3.5"
       env: CI_PIP_DEPS="Django>=1.10,<1.11"
-    - python: "3.6"
-      env: CI_PIP_DEPS="Django>=1.10,<1.11"
   allow_failures:
-    - python: "3.6"
-      env: CI_PIP_DEPS="Django>=1.10,<1.11"
+    - python: "2.7"
+      env: CI_PIP_DEPS="--pre Django"
 addons:
   apt:
     packages: