Switch from Travis CI to GitHub Actions (#64)

* Switch from Travis CI to GitHub Actions

Travis CI no longer works for ogórek and https://travis-ci.org/kisielk/og-rek
returns "not found".

-> Switch to GitHub Actions to fix CI.

We loose CI coverage for "go tip" as specifying "tip" Go version in
actions/setup-go is not supported out of the box:

https://github.com/actions/setup-go/issues/21

Maybe in 201x it was relevant to test wrt go tip, but nowdays I think
this is no longer much needed.

* CI += go1.16 - go1.22

Add CI coverage for all releases past go1.15 that we had before.

.github/workflows/ci.yml

+name: CI
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron:  '0 12 * * *' # Every day noon UTC
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        go:
+          - "1.7.x"
+          - "1.8.x"
+          - "1.9.x"
+          - "1.11.x"
+          - "1.12.x"
+          - "1.13.x"
+          - "1.14.x"
+          - "1.15.x"
+          - "1.16.x"
+          - "1.17.x"
+          - "1.18.x"
+          - "1.19.x"
+          - "1.20.x"
+          - "1.21.x"
+          - "1.22.x"
+      # Don't cancel everything when one Go version fails
+      fail-fast: false
+    runs-on: ubuntu-latest
+    steps:
+
+    - name: Install Go ${{ matrix.go }}
+      uses: actions/setup-go@v5
+      with:
+        go-version: ${{ matrix.go }}
+
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0 # Make "git describe" work
+
+    - run: go test

.travis.yml

-language: go
-
-go:
-  - 1.7
-  - 1.8
-  - 1.9
-  - "1.10"
-  - "1.11"
-  - "1.12"
-  - "1.13"
-  - "1.14"
-  - "1.15"
-  - tip

README.md

 ogórek
 ======
 [![GoDoc](https://godoc.org/github.com/kisielk/og-rek?status.svg)](https://godoc.org/github.com/kisielk/og-rek)
-[![Build Status](https://travis-ci.org/kisielk/og-rek.svg?branch=master)](https://travis-ci.org/kisielk/og-rek)
+[![Build Status](https://github.com/kisielk/og-rek/actions/workflows/ci.yml/badge.svg)](https://github.com/kisielk/og-rek/actions/workflows/ci.yml)
 
 ogórek is a Go library for encoding and decoding pickles.