Update get.sh to determine latest full release from Github

[johnmccabe]

Description

This PR updates the one-line installer get.sh to use the latest full release from Github rather than use a hard-coded version string.

Draft releases and prereleases are not returned by the endpoint used to determine the version.

Motivation and Context

Currently get.sh uses a hardcoded version, the script hosted on https://cli.openfaas.com pulls in an outdated version 0.4.5b

• I have raised an issue to propose this change

How Has This Been Tested?

Tested on Linux (Alpine, Fedora, Ubuntu, CentOS), and OSX.

For example:

$ sudo ./get.sh
Password:
Getting package https://github.com/alexellis/faas-cli/releases/download/0.4.7/faas-cli-darwin
Attemping to move faas-cli to /usr/local/bin
New version of faas-cli installed to /usr/local/bin

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

I think version pinning is more predictable which is why it's hard-coded, but let's try this suggestion to see how it goes.

[alexellis]

Thanks for the change. Merged and applied to server.