Add GitHub Action for CI

Migrating away from Travis CI because we [no longer get free builds :(](#222 (comment))

.github/workflows/blank.yml

@@ -0,0 +1,25 @@
+name: CI
+
+on:
+  push:
+    branches: 
+      - master
+  pull_request:
+    branches:
+      - master
+
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v2.1.5
+        with:
+          node-version: 12
+
+      - run: yarn install
+
+      - run: yarn test:ci && bash <(curl -s https://codecov.io/bash)