generate rspec html report in local env if COVERAGE=1
generate rspec html report in local env if COVERAGE=1

for people to use locally as they're working on a specific feature and
are wondering what the coverage looks like, the spec helper will now
react to COVERAGE=1 by generating an HTML report under coverage/

TEST PLAN
**** ****

run COVERAGE=1 bundle exec rspec path/to/spec.rb and verify you get a
legible report

Change-Id: I318dc2954b7e8824fc60200f98e2b0ab471d4fb4
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246637
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Charley Kline <05950d7e90ba5e3da323378368c22b3ffaaf6647@instructure.com>
QA-Review: Charley Kline <05950d7e90ba5e3da323378368c22b3ffaaf6647@instructure.com>
Product-Review: Charley Kline <05950d7e90ba5e3da323378368c22b3ffaaf6647@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms