resolves fakemongo#356 switch to GitHub Actions

Travis CI no longer offers Java < 9

Author: slonopotamus

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+on: [push, pull_request]
+jobs:
+  test:
+    strategy:
+      matrix:
+        java: [7, 8]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Java
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: Test
+        run: mvn clean install