Migrate to github actions

Travis-ci.org is going away. Migrate to github actions instead of
migrating over to travis-ci.com.

Signed-off-by: Billy Olsen <billy.olsen@gmail.com>

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: Java
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup JDK 11
+        uses: actions/setup-java@v2
+        with:
+          java-version: '11'
+          distribution: 'adopt'
+      - name: Ant build
+        run: ant -noinput -buildfile build.xml clean check jar unittest
+

README.md

@@ -1,6 +1,6 @@
 OpenRocket
 ==========
-[![Build Status](https://travis-ci.org/openrocket/openrocket.png) ](https://travis-ci.org/openrocket/openrocket)
+![Build Status](https://github.com/openrocket/openrocket/actions/workflows/build.yml/badge.svg)
 [![SourceSpy Dashboard](https://sourcespy.com/shield.svg)](https://sourcespy.com/github/openrocketopenrocket/)
 
 Overview