support brew install with --devel

Creating issue from comments made https://github.com/apache/incubator-openwhisk-cli/issues/153#issuecomment-459919832 by @alexkli

Both [wsk](https://github.com/Homebrew/homebrew-core/blob/master/Formula/wsk.rb) and [wskdeploy](https://github.com/Homebrew/homebrew-core/blob/master/Formula/wskdeploy.rb) are now available on homebrew. So I think this issue can be closed.

**However**, for `wsk` only the 6 month old 0.0.9-incubating version is available, while the [latest release](https://github.com/apache/incubator-openwhisk-cli/releases/tag/latest) isn't - and it includes some important new features like concurrency.

As a quick fix, I added a custom `wsk-latest` formula in my custom homebrew tap:

```
# only if you have wsk installed through homebrew
brew unlink wsk

brew tap alexkli/tap
brew install alexkli/tap/wsk-latest

wsk property get --cliversion
> whisk CLI version 2019-01-26T20:01:45.357+0000
```

But this is a bit annoying because of the `brew unlink` and switching back and forth between the two. With a separate formula, homebrew does not know that this is the same software, just a pre-release version.

IIUC, `latest` is not considered a stable release (e.g. it does not have a version number) and would be opt-in for early adopters. In Homebrew this can be done properly by adding a [devel version](https://docs.brew.sh/Formula-Cookbook#unstable-versions-devel-head) in the formula, that can be installed using `brew install --devel wsk`.

Would be cool if someone wants to tackle that - I could maybe in 2 weeks. And should that be a separate issue?