Switch from TravisCI to GitHub Actions (#257)

Also moves CI to use Java 11 instead of Java 8.

.github/workflows/build.yml

@@ -0,0 +1,20 @@
+name: build
+on: [push, pull_request]
+jobs:
+  gradle:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          distribution: temurin
+          java-version: 11
+
+      - name: Setup Gradle
+        uses: gradle/gradle-build-action@v2
+
+      - name: Execute Gradle build
+        run: ./gradlew -s build dokkaJar

README.md

@@ -3,7 +3,7 @@
 [![](https://img.shields.io/badge/project-libp2p-yellow.svg?style=flat-square)](https://libp2p.io/)
 [![Gitter](https://img.shields.io/gitter/room/libp2p/jvm-libp2p.svg)](https://gitter.im/jvm-libp2p/community)
 [![](https://img.shields.io/badge/freenode-%23libp2p-yellow.svg?style=flat-square)](http://webchat.freenode.net/?channels=%23libp2p)
-[![Build Status](https://travis-ci.com/libp2p/jvm-libp2p.svg?branch=master)](https://travis-ci.com/libp2p/jvm-libp2p)
+![Build Status](https://github.com/libp2p/jvm-libp2p/actions/workflows/build.yml/badge.svg?branch=master)
 [![Discourse posts](https://img.shields.io/discourse/https/discuss.libp2p.io/posts.svg)](https://discuss.libp2p.io)
 
 > a libp2p implementation for the JVM, written in Kotlin 🔥