Stricter/more useful doc builds in Travis

.travis.yml

@@ -11,9 +11,12 @@ install:
 script:
   # Main tests, with coverage!
   - coverage run --source=paramiko test.py --verbose
-  # Ensure documentation & invoke pipeline run OK
+  # Ensure documentation & invoke pipeline run OK.
-  - invoke www
+  # Run 'docs' first since its objects.inv is referred to by 'www'.
-  - invoke docs
+  # Also force warnings to be errors since most of them tend to be actual
+  # problems.
+  - invoke docs -o -W
+  - invoke www -o -W
 notifications:
   irc:
     channels: "irc.freenode.org#paramiko"