Use GitHub Actions

This replaces Travis CI which hasn't been working well for OCaml
projects recently.

.github/workflows/main.yml

@@ -0,0 +1,30 @@
+name: main
+
+on:
+  - pull_request
+  - push
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-latest
+        ocaml-compiler:
+          - 4.10.x
+          - 4.09.x
+          - 4.08.x
+          - 4.07.x
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+      - name: Use OCaml ${{ matrix.ocaml-compiler }}
+        uses: ocaml/setup-ocaml@v2
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+      - run: opam pin add ppx_factory.dev . --no-action
+      - run: opam depext ppx_factory --yes --with-doc --with-test
+      - run: opam install . --deps-only --with-doc --with-test
+      - run: opam exec -- dune build @default @runtest

README.md

@@ -1,5 +1,3 @@
-[![Build Status](https://travis-ci.org/cryptosense/ppx_factory.svg?branch=master)](https://travis-ci.org/cryptosense/ppx_factory)
-
 # ppx_factory
 
 `ppx_factory` is an OCaml preprocessor to derive factory methods and default values from type