Don't force primary db for applications collection

Now that there are no updates being performed to
applications through the API in owl, there is no need to
have the applications collection always read from the
primary. Anyway, it should never have been in all
cases, but only for reads for /applications.

Removing the ReadPreference statement means that the
read preference will default as per other collections;
processing hosts will read from the primary db, API hosts
will prefer a secondary db.
Merge pull request #38 from atlasapi/applications-read-slave

Don't force primary db for applications collection