Package migration message not needed on fresh ckan install

[davidread]

I was seeing seeing this in Travis:

Initialising the database...
You have no unmigrated package activities - you do not need torun migrate_package_activity.py.
Initialising DB: SUCCESS

and this PR gets rid of the middle line in this situation. This is simply a tidy up of something added in #3972.

Test with:

$ paster db clean
Cleaning DB: SUCCESS
$ paster db init
Initialising DB: SUCCESS

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply