Migrate to Github Actions (#245)

* Migrate to Github Actions
* Remove architecture param
* Remove Travis config, update readme

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: decimal-ci
+on: [push]
+jobs:
+  ci-job:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: [ '1.7.x', '1.15', '1.16', '1.17', '1.x' ]
+    name: Go ${{ matrix.go }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{ matrix.go }}
+      - run: go build .
+      - run: go test -v

README.md

@@ -1,6 +1,8 @@
 # decimal
 
-[![Build Status](https://app.travis-ci.com/shopspring/decimal.svg?branch=master)](https://app.travis-ci.com/shopspring/decimal) [![GoDoc](https://godoc.org/github.com/shopspring/decimal?status.svg)](https://godoc.org/github.com/shopspring/decimal) [![Go Report Card](https://goreportcard.com/badge/github.com/shopspring/decimal)](https://goreportcard.com/report/github.com/shopspring/decimal)
+[![Github Actions](https://github.com/shopspring/decimal/actions/workflows/ci.yml/badge.svg)](https://github.com/shopspring/decimal/actions/workflows/ci.yml)
+[![GoDoc](https://godoc.org/github.com/shopspring/decimal?status.svg)](https://godoc.org/github.com/shopspring/decimal) 
+[![Go Report Card](https://goreportcard.com/badge/github.com/shopspring/decimal)](https://goreportcard.com/report/github.com/shopspring/decimal)
 
 Arbitrary-precision fixed-point decimal numbers in go.