Travis was failing in 3.5 because there is a bug in fedora 3.5 that prevents
modification of the DC datastream.

Changed the test to test the MODS datastream instead so it will pass on 3.5.

See the converation here:
http://fedora-commons.1317035.n2.nabble.com/fcrepo-user-REST-API-modifyDatastream-on-DC-fails-with-InputStream-cannot-be-null-td7436589.html