spec: explicitly set host in files integration spec for rails 3

Change-Id: I2585fa63345902845450515dd95a517f02a0d5b6
Reviewed-on: https://gerrit.instructure.com/30641
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>