re-add httparty
re-add httparty

we're using this, but didn't explicitly have it in the Gemfile, so we lost
it with the testbot removal. not a problem in CI / canvas cloud, since
other plugins add it, but its absence would break canvas open source.

test plan:
1. remove any extraneous plugins
2. bin/spring stop
3. bundle install
4. bin/rails s
5. canvas should load up, instead of giving an httparty (or other) error

Change-Id: Ied85bb37720cca3815d80c9987a427d770fc6c2e
Reviewed-on: https://gerrit.instructure.com/65322
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
QA-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
Reviewed-by: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Tested-by: Jenkins

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