Check optional deps for being old as well

This way we can catch things like too old Mercurial.

Issue #511
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/requirements.py

@@ -224,7 +224,7 @@ def check_requirements():
     '''
     Performs check on requirements and raises an exception on error.
     '''
-    versions = get_versions()
+    versions = get_versions() + get_optional_versions()
     failure = False
 
     for version in versions: