default benchmark page is broken

[ansis]

mapbox-gl-js version: master

browser: all

Steps to Trigger Behavior

1. yarn run start-bench
2. open http://localhost:9966/bench/versions

Expected Behavior

Benchmarks run.

Actual Behavior

It tries loading https://s3.amazonaws.com/mapbox-gl-js/v1.7.0-dev/benchmarks.js and errors.

It looks like this is caused by us switching to -dev versions in package.json.

[arindam1993]

Along with the -dev postfix, I think the problem is its trying to find the benchmark build for the latest release, which is one increment behind whats in package.json.
The benchmark build for v1.7.0 doesn't exist on S3 yet because that tag hasn't been created yet
I have a quick fix that works ^^