Change the `importl10n` script to use the Beta channel, since Aurora is being removed

Since the Aurora channel is being removed, and is no longer going to be receiving updates (with the possible exception of security bugs), we need to change the target for the `gulp importl10n` command.

For more information, please refer to:
 - https://groups.google.com/forum/#!topic/firefox-dev/q4Eb5nEcfSg
 - https://hacks.mozilla.org/2017/04/simplifying-firefox-release-channels/
 - https://mozilla.github.io/process-releases/draft/development_overview/
 - http://release.mozilla.org/firefox/release/2017/04/17/Dawn-Project-FAQ.html

.github/CONTRIBUTING.md

@@ -8,7 +8,7 @@ The issue tracking system is designed to record a single technical problem. A bu
 
 If you are developing a custom solution, first check the examples at https://github.com/mozilla/pdf.js#learning and search existing issues. If this does not help, please prepare a short well-documented example that demonstrates the problem and make it accessible online on your website, JS Bin, GitHub, etc. before opening a new issue or contacting us on the IRC channel -- keep in mind that just code snippets won't help us troubleshoot the problem.
 
-Note that the translations for PDF.js in the `l10n` folder are synchronized with the Aurora branch of Mozilla Firefox. This means that we will only accept pull requests that add strings currently missing in the Aurora branch (because it will take at least six weeks before the most recent translations are in the Aurora branch), but keep in mind that the changes will be overwritten when we synchronize again.
+Note that the translations for PDF.js in the `l10n` folder are synchronized with the Beta branch of Mozilla Firefox. This means that we will only accept pull requests that add strings currently missing in the Beta branch (because it will take at least six weeks before the most recent translations are in the Beta branch), but keep in mind that the changes will be overwritten when we synchronize again.
 
 See also:
 - https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions

external/importL10n/locales.js

@@ -19,7 +19,7 @@ var fs = require('fs');
 var https = require('https');
 var path = require('path');
 
-// Defines all languages that have a translation at mozilla-aurora.
+// Defines all languages that have a translation at mozilla-beta.
 // This is used in make.js for the importl10n command.
 var langCodes = [
   'ach', 'af', 'ak', 'an', 'ar', 'as', 'ast', 'az', 'be', 'bg',
@@ -44,10 +44,10 @@ function downloadLanguageFiles(root, langCode, callback) {
   console.log('Downloading ' + langCode + '...');
 
   // Constants for constructing the URLs. Translations are taken from the
-  // Aurora channel as those are the most recent ones. The Nightly channel
+  // Beta channel as those are the most recent ones. The Nightly channel
   // does not provide all translations.
-  var MOZ_AURORA_ROOT = 'https://hg.mozilla.org/releases/l10n/mozilla-aurora/';
+  var MOZ_BETA_ROOT = 'https://hg.mozilla.org/releases/l10n/mozilla-beta/';
-  var MOZ_AURORA_PDFJS_DIR = '/raw-file/tip/browser/pdfviewer/';
+  var MOZ_BETA_PDFJS_DIR = '/raw-file/tip/browser/pdfviewer/';
 
   // Defines which files to download for each language.
   var files = ['chrome.properties', 'viewer.properties'];
@@ -61,7 +61,7 @@ function downloadLanguageFiles(root, langCode, callback) {
   // Download the necessary files for this language.
   files.forEach(function(fileName) {
     var outputPath = path.join(outputDir, fileName);
-    var url = MOZ_AURORA_ROOT + langCode + MOZ_AURORA_PDFJS_DIR + fileName;
+    var url = MOZ_BETA_ROOT + langCode + MOZ_BETA_PDFJS_DIR + fileName;
 
     https.get(url, function(response) {
       var content = '';