Set the default ruby in the RVM image to 2.4
Set the default ruby in the RVM image to 2.4

Change-Id: Ied92e26ac4aa875ab739785cd280e0247425bbe3
Reviewed-on: https://gerrit.instructure.com/113358
Reviewed-by: Mark Severson <cdf1f90aee9d0d073a2a24141ebc71887f3ba533@instructure.com>
Product-Review: Tyler Pickett <3f20fcf21b1970c94966160adcf831eedd808a06@instructure.com>
QA-Review: Tyler Pickett <3f20fcf21b1970c94966160adcf831eedd808a06@instructure.com>
Tested-by: Tyler Pickett <3f20fcf21b1970c94966160adcf831eedd808a06@instructure.com>

instructure/dockerfiles,instructure/dockerfiles,instructure/dockerfiles,instructure/dockerfiles