support brew install with --devel

[rabbah]

Creating issue from comments made #153 (comment) by @alexkli

Both wsk and wskdeploy 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 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 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?