Gemspec for Academic Benchmarks gem uses a URI

Fixes CNVS-15705

This 'just' needed a valid uri to keep some versions of bundler from
throwing an invalid gemspec error, which could prevent bins or native
extensions from getting installed, causing problems in extreme cases.

Test plan:

Run `bundle install` and see that you don't get an error about an
invalid gemspec (using an impacted version of bundler, such as 1.3.2.

Alternative, just trust me that this fixes the error, don't worry about
changing your bundler version, and simply verify that Canvas
still bundles.

Change-Id: Ibd658c55e47118355f5bed516f23c627edf1d153
Reviewed-on: https://gerrit.instructure.com/41482
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>