CLI install docs should prefer omnibus packages over gem install

[SpComb]

The Quick Start docs only link to the omnibus release packages for OSX, but these are now also available as Debian/Ubuntu deb packages for the 1.4 release. The docs for Linux should be updated to prefer the omnibus deb package over a manual gem install.

Problems with the manual gem install:

• Ubuntu: gem install kontena-cli doesn't actually work: you need to either sudo gem install, or gem install --user-install and get ~/.gem/ruby/2.3.0/bin onto your $PATH: Improved instructions for Kontena-CLI in Ubuntu  kontena#2604
• The 1.4 release now includes native extensions in the kontena-cli dependencies, so gem install will fail if you do not have a compiler + ruby-dev installed. Is that available by default on normal Ubuntu/Windows ruby installs?: The native websocket_mask extension should be an optional dependency kontena-websocket-client#11

It's still unclear what we should do about Windows, because we don't have a omnibus packages for that platform (kontena/kontena#2307). Should we recommend Windows Subsystem for Linux + the deb package for Windows 10, or do the windows ruby distributions include the necessary compiler + ruby-dev stuff to install native extensions?

[SpComb]

For Ubuntu, signed CLI packages are also available on bintray as an APT repository. Downloading and installing the deb is easier and faster for new users, but the signed packages from the APT repo are better for long-term use.

[jakolehm]

I think we should recommend WSFL for Windows users and instruct Ruby + gem just as a fallback if user cannot use WSFL.

[jakolehm]

Docs also link now just to Github release page which is not very user friendly. Maybe we could provide some wrapper (microservice) that allows user to download latest omnibus package directly?