make rails 4.2 the default
make rails 4.2 the default

can explicitly switch back with CANVAS_RAILS4_2=0

Change-Id: I5dc5b0146028bafc9c152cb9909b287494d0ed2e
Reviewed-on: https://gerrit.instructure.com/92882
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

HotChalk/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms