expect django greater than 2, and use open() instead of file() call

.travis.yml

@@ -2,7 +2,7 @@ language: python
 python:
   - "3.6"
 env:
-  - DJANGO="Django>=2"
+  - DJANGO="Django>2.1"
 install:
   - pip install -q $DJANGO
   - pip install -r requirements.txt

binder/settings.py

@@ -70,7 +70,7 @@ except IOError:
     try:
         from random import choice
         SECRET_KEY = ''.join([choice('abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)') for i in range(50)])
-        secret = file(SECRET_FILE, 'w')
+        secret = open(SECRET_FILE, 'w')
         secret.write(SECRET_KEY)
         secret.close()
     except IOError: