Pin version of snowballstemmer

This was relied on by one of our other dependencies (likely through a more
extensive dependency chain).  It updated its most recent version to 3.0.0 today
and that broke our documentation tests, so pin to an older version for now while
I figure out:
1. what package is using it
2. whether it's necessary to be using that dependency

----
Signed-off-by: Lydia Duncan <[email protected]>

Author: lydia-duncan

docs-requirements.txt

@@ -1 +1,2 @@
 sphinx_rtd_theme==2.0.0
+snowballstemmer==2.2.0