Add support for Ruby 2.1 (ruby#71)

Also include the commit in the cache key for truffleruby-head

* That way new caches can be created if the previous cache is for a different commit.

Allow running external function before 'bundle install'

External actions may use setup-ruby, and some actions make changes/additions to the build environment. These may be needed for dependencies.

Hence, allow those external actions to run code before 'bundle install'. This is done by passing a function as a parameter to setupRuby().

Implement automatic bundle install and caching

* Not yet enabled by default.

Add windows-2016 to supported platforms

* Recommend to use *-latest.

Add support for Ubuntu 20.04

Add TruffleRuby 20.1.0

Add an exported setupRuby() function which can be called from other a…

…ctions

* Inputs are passed as simple object properties.
* It is recommended to expose the same inputs for other actions.
* See ruby#58 and ruby#33.

Use Bundler 1 on Ruby 2.3

* At least until there is a new Bundler 2 release fixing this.
* Fixes ruby#51

Fix checks for head versions

* Always use common.isHeadVersion().