Remove chromedriver-helper and use system binaries

[benthorner]

Previously we used the chromedriver-helper gem to automatically download
a suitable chromedriver binary for use with headless testing. This gem
is no longer maintained, and since work had already begun to start using
the system binary, it seems best to drop it altogether.

flavorjones/chromedriver-helper#83

A system binary for chromedriver should already installed on all Jenkins
agents, but a Puppet gem is still needed to install it on all Dev VMs.

[benthorner]

One possible issue with this is that an older version of chromedriver-helper used to add it's own chromedriver binstub, which is still present on older machines. This means running bundle exec chromedriver will try to use the old binstub, which won't work if the gem isn't there anymore; the easiest thing to do is just to manually find and remove the old binstub, as bundler doesn't clean them up - this stops it getting picked-up on the path and lets the system one be used as expected.

Example of removing: ~/.rbenv/versions/2.5.3/bin/chromedriver

[benthorner]

Closing this in favour of #18. The nice thing about webdrivers is that it automatically updates chromedriver when a new version of Chrome is released, so it's definitely worth using to avoid the faff of doing this manually every time.