Allow NULL values in the field comment on table jenkins_build_changesets

This PR aims to solve the issue detailed on issue #8 

This could be done using a new migration, however since the strategy in other PRs is editing the original migration file, I did the same.

In a running system, to apply this change without having to delete all tables is to run a script directly in the database:
(i.e. Mysql => ALTER TABLE jenkins_build_changesets CHANGE comment LONGTEXT NULL)

If you think this should be solved with a new migration, let me know, and I will send you a new PR.