Ruby 1.9.2+ compatibility: Fix require statement

Running the rakefile with Ruby 1.9.2 or newer will result in the following failure:

````
rake aborted!
cannot load such file -- config/environment
/apps/redmine-2.2.2/lib/tasks/migrate_jira.rake:4:in `require'
/apps/redmine-2.2.2/lib/tasks/migrate_jira.rake:4:in `<top (required)>'
/apps/redmine-2.2.2/Rakefile:7:in `<top (required)>'
````

The reason is that, for security reasons, the pwd is no longer included in Ruby's `LOAD_PATH`.

By the way, thanks for setting up a Github repo! It's much easier to work with than a Redmine ticket.