[CD/CI] Add GitHub Actions to run tests and publish new package versi…

…ons to npm (#683)

Add GitHub Actions to:
1. Run tests for every PR and every commit to `master`.
2. Publish a new package version to npm every time we create a new release.

Additionally, updated the gulp test scripts in the following way:
* `gulp test:integration` does not run unit tests anymore, just the integration tests.
* `gulp test` now runs integration tests in addition to the unit and browser tests it was already running before.
* Browser tests now run in FirefoxHeadless and ChromeHedless because the Firefox browser we used in travis won't work for GH Actions.

* Missing a workflow to update gh-pages when we release a new version.

gulpfile.js

@@ -158,8 +158,6 @@ gulp.task(
   gulp.series('build:node', 'test:init-istanbul', function testIntegration() {
     return gulp
       .src([
-        'test/test-nodejs.js',
-        'test/unit/**/*.js',
         'test/integration/**/*.js'
       ])
       .pipe(
@@ -179,7 +177,7 @@ gulp.task('build', gulp.series('clean', 'build:node', 'build:browser'));
 
 gulp.task(
   'test',
-  gulp.series('clean', 'test:unit', 'test:browser', function test(done) {
+  gulp.series('clean', 'test:unit', 'test:browser', 'test:integration', function test(done) {
     done();
   })
 );

karma.conf.js

@@ -6,7 +6,7 @@ delete webpackConfig.output;
 module.exports = function(config) {
   config.set({
     frameworks: ["mocha", "chai-as-promised", "chai", "sinon"],
-    browsers: ["Firefox"],
+    browsers: ["FirefoxHeadless", "ChromeHeadless"],
 
     files: [
       "dist/stellar-sdk.js",