Merge pull request #1561 from incuna/i18n

Mark strings in AuthTokenSerializer as translatable 

rest_framework/authtoken/serializers.py

 from django.contrib.auth import authenticate
+from django.utils.translation import ugettext_lazy as _
+
 from rest_framework import serializers
 
 
@@ -15,10 +17,13 @@ class AuthTokenSerializer(serializers.Serializer):
 
             if user:
                 if not user.is_active:
-                    raise serializers.ValidationError('User account is disabled.')
+                    msg = _('User account is disabled.')
+                    raise serializers.ValidationError(msg)
                 attrs['user'] = user
                 return attrs
             else:
-                raise serializers.ValidationError('Unable to login with provided credentials.')
+                msg = _('Unable to login with provided credentials.')
+                raise serializers.ValidationError(msg)
         else:
-            raise serializers.ValidationError('Must include "username" and "password"')
+            msg = _('Must include "username" and "password"')
+            raise serializers.ValidationError(msg)