Run tests on gh actions

Related #60.

Can't figure out what's wrong with travis, but gh actions seem to work.
Migrated travis.yml to runtest.yml.

coveralls fails for me because I don't have the account there, but it's
reporting coverage amounts before it fails to upload.

Tests against multiple versions of lua using leafo's matrix.luaversion
with roughly the same versions as travis.yml.

What's missing
==============

Notification setup from travis is missing. Github sends email
notifications. You can choose:
 * Email
 * Web
 * Send notifications for failed workflows only
There are third-party irc gh actions, but I don't see how you'd do
`on_success: change` in an action.

luacheck was disabled in travis.yml, so it's disabled here too.

.github/workflows/runtest.yml

@@ -0,0 +1,43 @@
+name: Validate Code
+
+on:
+  pull_request:
+    branches:
+    - master
+    - refactor
+  push:
+    branches:
+    - master
+    - refactor
+
+jobs:
+  test:
+    name: Run Tests
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        luaVersion: ["5.1.5", "luajit-2.0.5", "luajit-2.1.0-beta3"]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Lua
+      uses: leafo/gh-actions-lua@v8.0.0
+      with:
+        luaVersion: ${{ matrix.luaVersion }}
+    - name: Setup Lua Rocks
+      uses: leafo/gh-actions-luarocks@v4
+    - name: Install dependencies
+      run: |
+        luarocks --local install busted
+        luarocks --local install luacov
+        luarocks --local install luacov-coveralls
+    - name: Run busted
+      run: ~/.luarocks/bin/busted --verbose --coverage spec
+    - name: Upload coverage
+      run: |
+        # ignore dotfile directories created by lua setup
+        ~/.luarocks/bin/luacov-coveralls --exclude '^%.%a+$'
+    # - name: Run luacheck
+    #   run: luacheck --std max+busted *.lua spec
+