Enable org-journal-mode before using outline.el defuns

When creating a new org-journal-entry, if org-mode or outline mode
haven't already been enabled, then you will receive the error:

Symbol's function definition is void hide-sublevels:

In order to fix this, we enable org-journal-mode prior to calling
hide-sublevels
Merge pull request #21 from keelerm84/bug/hide-sublevels

Enable org-journal-mode before using outline.el defuns