Add CI flow for tests on Github actions

Travis CI has been down for a while now and we'd like to switch
to Github Actions. This change introduces a new CI build on Github
Actions that runs tests on Ruby implementations and also mark
some Ruby failing implementations as experimental so the build
still passes.

It does not contain all the requirements from the previous Travis
CI yet, but it's a good start.

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    continue-on-error: ${{ matrix.experimental }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [2.7, jruby, jruby-head, truffleruby-head]
+        experimental: [false]
+        include:
+          - ruby: head
+            experimental: true
+          - ruby: truffleruby
+            experimental: true
+
+    env:
+      JAVA_OPTS: '-Xmx1024m'
+      RUBYOPT: '-w'
+      JRUBY_OPTS: '--dev'
+
+    name: "Tests: Ruby ${{ matrix.ruby }}"
+    steps:
+    - name: Clone Repo
+      uses: actions/checkout@v2
+    - name: Setup system Ruby ${{ matrix.ruby }}
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - name: Run tests
+      run: |
+        gem install bundler --version 1.17.3
+        echo JAVA_OPTS: $JAVA_OPTS
+        bundle exec rake ci