Revert to older version of TinyMCE.

We had trouble with Cloudfront/hashed files and need more time
to investigate.
This reverts commit e8f53ac9dc6d8a1d9928787635c7c5f7e409a9ec.
Merge pull request #3067 from edx/christina/revert_tinymce

Revert to older version of TinyMCE.