Do not prompt to update to the latest stable version if Fiji is used

The registry will still be contacted if the user has allowed it, but
repeated prompts to upgrade will no longer appear in Fiji.  The upgrade
prompts should remain in plain ImageJ.
Add 'isFiji()' method and remove 'IS_FIJI' constant

This prevents the Fiji-ness from being detected incorrectly if the
Updater class is loaded before the ImageJ instance is created.
Merge pull request #788 from melissalinkert/fiji-update-check

Do not prompt to update to the latest stable version if Fiji is used