BibIndex: tag.recjsonvalue NOT NULL

* Complements upgrade recipe invenio_2014_08_13_tag_recjsonvalue by
  adding the further property NOT NULL to the tag.recjsonvalue column
  definition.
  (closes #1947)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Some quick notes on this:
  - the backquote technique is OK, the create table statements are quite stable across MySQL versions, but sure now that we are moving towards PostgreSQL, we may start improving these;
  - `try/except` is good, but let's not forget to analyse all its output carefully not to have any masking; 
  - we could also study `SELECT ... INFORMATION_SCHEMA ...` and friends before calling `ALTER`.