Add stable URL:s to the latest releases

[NiklasBr]

Summary

Releases at https://download.newrelic.com/php_agent/release/ are tagged with their versions in the URL, which in many ways is nice, but it also makes it very hard to write a script which can automatically fetch the latest version.

You either have to download the HTML and grep through the HTML as text hoping to find what you need, or do client-side DOM parsing.

Desired Behavior

What if there was a symlink (or equivalent) to the latest version kind of like this:

Additional context

Got the idea while trying to resolve a suggestion in #706 .

For example ImageMagick does this: https://imagemagick.org/script/mirror.php or Redis which has https://download.redis.io/redis-stable.tar.gz as a consistently relevant URL.