Only show imminent DeprecationWarnings by default

Otherwise the PendingDeprecationWarnings for the next version of Django
become very noisy. The PendingDeprecationWarnings are generally ignored
because of the large range of Django versions supported, so printing
them by default causes nothing but noise. This causes actual warnings to
be ignored.

runtests.py

@@ -1,20 +1,21 @@
 #!/usr/bin/env python
 
-import sys
+from __future__ import absolute_import, unicode_literals
+
+import argparse
 import os
 import shutil
+import sys
 import warnings
-import argparse
 
 from django.core.management import execute_from_command_line
 
-
 os.environ['DJANGO_SETTINGS_MODULE'] = 'wagtail.tests.settings'
 
 
 def make_parser():
     parser = argparse.ArgumentParser()
-    parser.add_argument('--deprecation', choices=['all', 'pending', 'imminent', 'none'], default='pending')
+    parser.add_argument('--deprecation', choices=['all', 'pending', 'imminent', 'none'], default='imminent')
     parser.add_argument('--postgres', action='store_true')
     parser.add_argument('--elasticsearch', action='store_true')
     parser.add_argument('--elasticsearch2', action='store_true')