Add Github actions file

Remove Travis CI file. Travis recently announced that in a few weeks all
free tier projects will run out of build time. Which is quite annoying,
since I just got this project working on Travis.

Move to Github actions, since they claim "always free" for public
projects. And they were super fast to get working. And the jobs run so
much faster than on Travis.

Add test and docs badges to README.

.github/workflows/tests.yml

@@ -0,0 +1,27 @@
+name: tests
+on:
+  push:
+    paths-ignore:
+      - 'docs/**'
+  pull_request:
+jobs:
+  tests:
+    strategy:
+      matrix:
+        os:
+          - ubuntu-16.04
+          - ubuntu-18.04
+          - ubuntu-20.04
+          - macos-10.15
+          - macos-11.0
+          - windows-latest
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 1
+      - name: Install zsh
+        run: sudo apt-get --yes install zsh
+        if: runner.os == 'Linux'
+      - run: ./tests.sh
+        shell: bash

README.md

@@ -1,6 +1,9 @@
 Liquid Prompt — a useful adaptive prompt for Bash & zsh
 =======================================================
 
+![Tests](https://github.com/nojhan/liquidprompt/workflows/tests/badge.svg)
+[![Documentation Status](https://readthedocs.org/projects/liquidprompt/badge/?version=stable)](https://liquidprompt.readthedocs.io/)
+
 Liquid Prompt gives you a nicely displayed prompt with useful information
 when you need it. It shows you what you need when you need it.
 You will notice what changes *when* it changes, saving time and frustration.