Added missing file reference in .app

This fixes rebar compile failing

    $ rebar --version
    rebar 2.2.0 R16B02 20140407_103552 No VCS info available.
    # I also found this with R17 though

    $ rebar compile
    ==> emongo (compile)
    Compiled src/emongo_utils.erl
    Compiled src/emongo_app.erl
    Compiled src/emongo_packet.erl
    Compiled src/emongo_conn.erl
    Compiled src/emongo_bson.erl
    Compiled src/emongo.erl
    ERROR: One or more .beam files exist that are not listed in emongo.app:
    	* emongo_utils
    ERROR: compile failed while processing /home/ubuntu/*****/deps/emongo: rebar_abort