bump eventmachine for ruby 2.2 support

Change-Id: Ifdee2ccf72083a8f273c71f0330a76e32d3c6c18
Reviewed-on: https://gerrit.instructure.com/46319
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>