Get rid of gdk import warning.
When running mopidy in headless mode we keep getting a gdk import error that can safely be ignored. Ideally we should find a way to suppress this warning.

diff --git a/mopidy/frontends/mpris/__init__.py b/mopidy/frontends/mpris/__init__.py
index 813e03a2..5be9c6cf 100644
--- a/mopidy/frontends/mpris/__init__.py
+++ b/mopidy/frontends/mpris/__init__.py
@@ -1,5 +1,7 @@
 from __future__ import unicode_literals
 
+import os
+
 import mopidy
 from mopidy import exceptions, ext
 from mopidy.utils import formatting, config
@@ -89,6 +91,10 @@ class Extension(ext.Extension):
         return schema
 
     def validate_environment(self):
+        if 'DISPLAY' not in os.environ:
+            raise exceptions.ExtensionError(
+                'An X11 $DISPLAY is needed to use D-Bus')
+
         try:
             import dbus  # noqa
         except ImportError as e:
diff --git a/mopidy/frontends/mpris/actor.py b/mopidy/frontends/mpris/actor.py
index 92805bd3..fae8618f 100644
--- a/mopidy/frontends/mpris/actor.py
+++ b/mopidy/frontends/mpris/actor.py
@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import logging
+import os
 
 import pykka
 
@@ -10,10 +11,14 @@ from mopidy.frontends.mpris import objects
 logger = logging.getLogger('mopidy.frontends.mpris')
 
 try:
-    import indicate
-except ImportError as import_error:
-    indicate = None  # noqa
-    logger.debug('Startup notification will not be sent (%s)', import_error)
+    indicate = None
+    if 'DISPLAY' in os.environ:
+        import indicate
+except ImportError:
+    pass
+
+if indicate is None:
+    logger.debug('Startup notification will not be sent')
 
 
 class MprisFrontend(pykka.ThreadingActor, CoreListener):
['mopidy/frontends/mpris/actor.py', 'mopidy/frontends/mpris/__init__.py']