Bump dependencies (codeclimate#87)

* Bump Ruby to 2.3.3

* Bump Rubocop and other gems

* Stop creating blank files

* Fix code style

Add basic DEVELOPERS.md

Use default fingerprinting for Metrics/BlockLength

Noticed in a recent build that this cop is showing up as fixed/new
rather than changed. It was added in RuboCop v0.44, which we just
upgraded to.

Add GEM_PATH env var to Dockerfile. (codeclimate#78)

Upgrade Rubocop to v0.44.1

Upgrade Rubocop to v0.44.1

use engine-cache v3

new engine-cache

use the engine-cache

Support automatic channel building for rubocop (codeclimate#75)