update .gitignore for rails 5.1
update .gitignore for rails 5.1

Change-Id: I690f48fa4975ce7c1f8cbe92ae7fbcea221400a2
Reviewed-on: https://gerrit.instructure.com/120758
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms