Switch to github actions

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: signal-cli CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        java: [ '1.8', '12.0.2' ]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up JDK
+      uses: actions/setup-java@v1
+      with:
+        java-version: ${{ matrix.java }}
+    - name: Build with Gradle
+      run: ./gradlew build

.travis.yml

@@ -1,8 +0,0 @@
-language: java
-before_cache:
-  - rm -f  $HOME/.gradle/caches/modules-2/modules-2.lock
-  - rm -fr $HOME/.gradle/caches/*/plugin-resolution/
-cache:
-  directories:
-    - $HOME/.gradle/caches/
-    - $HOME/.gradle/wrapper/