[#6342] bump markdown version, invalidate markdown cache, for small fix.

Would be good to upgrade much further, but there are various regressions,
mostly with markdown internal placeholders showing up in the output, which is
no good.  This starts at 2.3 and perhaps is because of our own extensions
needing update, but also I think due to handling invalid markdown (e.g.
markdown within html) differently -- but I don't want that to break.  More info
at https://github.com/Python-Markdown/markdown/issues/458 and
https://github.com/Python-Markdown/markdown/issues/222
diff --git a/Allura/allura/lib/app_globals.py b/Allura/allura/lib/app_globals.py
index 749edda..883634d 100644
--- a/Allura/allura/lib/app_globals.py
+++ b/Allura/allura/lib/app_globals.py
@@ -105,7 +105,7 @@
                 field_name, artifact.__class__.__name__)
             return self.convert(source_text)
 
-        bugfix_rev = 3  # increment this if we need all caches to invalidated (e.g. xss in markdown rendering fixed)
+        bugfix_rev = 4  # increment this if we need all caches to invalidated (e.g. xss in markdown rendering fixed)
         md5 = None
         # If a cached version exists and it is valid, return it.
         if cache.md5 is not None:
diff --git a/requirements.txt b/requirements.txt
index 7eb11fd..eba6442 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -20,7 +20,7 @@
 httplib2==0.7.4
 iso8601==0.1.4
 Jinja2==2.9.6
-Markdown==2.2.0
+Markdown==2.2.1
 Ming==0.5.5
 oauth2==1.5.170
 # tg2 dep PasteDeploy must specified before TurboGears2, to avoid a version/allow-hosts problem