Mention the minimum required bob.db.base version

The bob.db.base.FileDatabase class was introduced in bob.db.base 2.2.1:

https://github.com/bioidiap/bob.db.base/commit/b3e2273d3243a7c4adaded4129c50a86927f2724

conda/meta.yaml

@@ -34,7 +34,7 @@ requirements:
     - setuptools {{ setuptools }}
     - bob.blitz
     - bob.core
-    - bob.db.base
+    - bob.db.base >2.2.0
     - bob.extension
     - bob.io.base
     - bob.learn.activation