Migrate TravisCI to GitHub Actions

This repo hasn't received much attention recently. As such, the TravisCI
yaml file was referencing
[EOL](https://www.ruby-lang.org/en/downloads/branches/) versions of
ruby. Moreover, TravisCI itself isn't generally used for open source
anymore. There are heavy restrictions on build minutes, as noted
[here](jekyll/jekyll#8492) in the core Jekyll
project.

This PR does the following:

 * Removes `.travis.yml`. We won't run jobs on TravisCI anymore.
 * Replaces it with `.github/workflows/ci.yml`. We'll start running CI
   on GitHub Actions.
 * Updates Ruby versions to those currently supported. This matches the
   ones tested in the [core jekyll project](https://github.com/jekyll/jekyll/blob/796ae15c31147d1980662744ef0f19a15a27cdee/.github/workflows/ci.yml#L20-L28).

This is making progress toward jekyll#192. I plan to work toward supporting
Ruby 3 and Numo in classifier-reborn, but the first step is getting CI
working with the existing code.

Author: mkasberg

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: Continuous Integration
+
+on:
+  push:
+    branches:
+      - master
+      - "*-stable"
+  pull_request:
+    branches:
+      - master
+      - "*-stable"
+
+jobs:
+  ci:
+    name: "Run Tests (${{ matrix.label }})"
+    runs-on: "ubuntu-latest"
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - label: Ruby 2.7
+            ruby_version: "2.7"
+          - label: Ruby 3.0
+            ruby_version: "3.0"
+          - label: Ruby 3.1
+            ruby_version: "3.1"
+          - label: JRuby 9.3.4.0
+            ruby_version: "jruby-9.3.4.0"
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v3
+      - name: "Set up ${{ matrix.label }}"
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby_version }}
+          bundler-cache: true
+      - name: Run Minitest based tests
+        run: bash script/test