Pass symbolic option to config_var endpoint

When passed, symbolic representations of attached resources are rendered
instead of their actual values.  One also requires a feature flag to see
this.
Re-bump Excon to 0.40.0
Merge pull request #1227 from heroku/brandur-bump-excon-2

(Re-)bump Excon to 0.40.0
Merge pull request #1226 from fdr/symbol

Pass symbolic option to config_var endpoint
hide http git for now
use variables to find http git host