nio4r 2.0.0 primarily includes new features and bugfixes, with few breaking
changes. The primary reason for bumping the major version is dropping support
for all Ruby versions prior to 2.2.2, so as to match Rails 5.

Full release announcement here:

https://groups.google.com/forum/#!topic/socketry/ZDIUj1ufiJ8

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Apps that depend on Action Cable don't need Blade for app development,
so we can remove the gem dependency.

We do need Blade for Action Cable dev, so we bundle it in the Gemfile.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

None of the other components use them, so we should be consistent.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

- adapter -> pubsub (re)rename internally
- Change variable names to match method names
- Add EventMachine `~> 1.0` as a runtime dependency of ActionCable
- Refactor dependency loading for adapters

This reverts commit d0393fcc, reversing
changes made to 3b7ccadf.

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```

The issue: https://github.com/celluloid/celluloid/issues/637