Fix configure checks requiring module loading

After 98db40b the check outputs:

    $ erl -noshell -eval 'code:load_file(file), io:format("~p~n",[[erlang:function_exported(file,sendfile,5)]]), erlang:halt().'
    [true]

    =ERROR REPORT==== 25-Jun-2013::09:40:51 ===
    Can't load module that resides in sticky dir

Use code:ensure_loaded/1 instead as suggested by Tuncer Ayaz. Fix similar
module loading check in rebar.config.script.