Pin version of snowballstemmer (#101)

[no objections from the team, not reviewed]

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

Author: lydia-duncan

docs-requirements.txt

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