[Dev setup] Add --no-gem-cache option to ./dev setup-unit-tests

For some dev setups there's no need to use a gem cache - they can
directly install gems from rubygems.org. This simplifies and speeds up
the dev setup process.

Usage:

````
./dev setup-unit-tests --no-gem-cache
````
[Dev setup] Add warning about using --no-gem-cache

As requested by Victor during pull request review.
Merge pull request #1667 from jamestyj/jamestyj-no-gem-cache

[Dev setup] Add --no-gem-cache option to ./dev setup-unit-tests