Replace Travis CI with Github Actions

* Add test workflow with Go matrix

.github/workflows/test.yaml

@@ -0,0 +1,30 @@
+name: test
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+jobs:
+  build:
+    name: go
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        go: ['1.13', '1.14', '1.15']
+    steps:
+      - name: setup
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{matrix.go}}
+
+      - name: checkout
+        uses: actions/checkout@v2
+
+      - name: tools
+        run: go get golang.org/x/lint/golint
+
+      - name: test
+        run: make

README.md

@@ -1,4 +1,5 @@
-# Sling [![Build Status](https://travis-ci.org/dghubble/sling.svg?branch=master)](https://travis-ci.org/dghubble/sling) [![GoDoc](https://godoc.org/github.com/dghubble/sling?status.svg)](https://godoc.org/github.com/dghubble/sling)
+# Sling [![Build Status](https://github.com/dghubble/sling/workflows/test/badge.svg)](https://github.com/dghubble/oauth1/actions?query=workflow%3Atest+branch%3Amaster) [![Coverage](https://gocover.io/_badge/github.com/dghubble/sling)](https://gocover.io/github.com/dghubble/sling) [![GoDoc](https://godoc.org/github.com/dghubble/sling?status.svg)](https://godoc.org/github.com/dghubble/sling)
+
 <img align="right" src="https://storage.googleapis.com/dghubble/small-gopher-with-sling.png">
 
 Sling is a Go HTTP client library for creating and sending API requests.