Add GitHub actions.

[toastkidjp]

See

https://github.com/ohmae/touch-icon-extractor/blob/develop/.github/workflows/ci.yml

name: ci

on: [push]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v1
    - name: set up JDK 1.8
      uses: actions/setup-java@v1
      with:
        java-version: 1.8
    - name: Run unit test
      run: |
        chmod +x gradlew
        ./gradlew testDebugUnitTest