Align ci workflow (#22)

* Align CI workflow

We want an improved aligned workflow setup for all
our postgres extensions.
This workflow will run tests after pushing commits into
the main branch and into any open pull request's branch.
It runs tests on PostgreSQL Versions 10 to 15.

One important thing here is that the workflow uses pgxn/pgxn-tools
Docker image. It installs specified version of PostgreSQL and run tests
by pg-build-test, which runs make installcheck under the hood.

* add ci badge to README.md

    Let anyone see directly the current build status

.github/workflows/main.yml

@@ -0,0 +1,22 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+      - main
+  pull_request:
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        pg: [15, 14, 13, 12, 11, 10]
+    name: 🐘 PostgreSQL ${{ matrix.pg }}
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+      - run: pg-start ${{ matrix.pg }}
+      - uses: actions/checkout@v2
+      - run: pg-build-test
+

README.md

@@ -1,5 +1,6 @@
-## adevens ltree implementation
-[![Build Status](https://travis-ci.org/adeven/ltree.png?branch=master)](https://travis-ci.org/adeven/ltree)
+[![CI](https://github.com/adjust/wltree/actions/workflows/main.yml/badge.svg)](https://github.com/adjust/wltree/actions/workflows/main.yml)
+
+## adjust ltree implementation
 
 This is adevens patched version of Postgres ltree module.