Explanation

Travis CI builds are failing due to network issues downloading CKEditor libraries. This PR switches the source of those libraries to GitHub for better reliability, and in the process removes an unneeded download.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The linter/Karma presubmit checks have passed.
• The PR is made from a branch that's not called "develop".
• The PR has an appropriate "PROJECT: ..." label (Please add this label for the first-pass review of the PR).
• The PR has an appropriate "CHANGELOG: ..." label (If you are unsure of which label to add, ask the reviewers for guidance).
• The PR follows the style guide.
• The PR addresses the points mentioned in the codeowner checks for the files/folders changed. (See the codeowner's wiki page.)
• The PR is assigned to an appropriate reviewer.