Add support for Ruby 2.1

• Add support for Ruby 2.1 (#70, #71)

Treat error from restoring the cache as a cache miss

• Treat error from restoring the cache as a cache miss (#68, #66)
• Update README to reflect conditions for self-hosted runners (#67)
• Also include the commit in the cache key for truffleruby-head.

Add JRuby 9.2.12.0

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().

Support automatic Bundler caching as opt-in

• See #52.

Add support for the windows-2016 platform

• Fixes #61.
• See #60.

Add support for Ubuntu 20.04

• Fixes ruby/ruby-builder#2

Add TruffleRuby 20.1.0

v1.35.0

Add TruffleRuby 20.1.0

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

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

Use Bundler 1 on Ruby 2.3

• At least until there is a new Bundler 2 release fixing ruby/rubygems#3570.
• Fixes #51

Fix detection of head version when using ruby-version: debug

Fix checks for head versions

* Always use common.isHeadVersion().