Update CKEditor library to 4.6.2

Update to CKEditor 4.6.2: http://ckeditor.com/blog/CKEditor-4.6.2-released

4.6.2 brings:

1. new default skin: Moono-Lisa, compared to the previous default of Moono. We should consider adopting it. We have #2831442: [PP-1] Consider switching to CKEditor 4.6's new "Moono-Lisa" skin for that.
2. Paste from Word: rewritten from scratch, vastly improved
3. accessibility: keyboard shortcuts now displayed in toolbar button tooltips and context menu entries
4. … other improvements, including one worth calling out here: a config.image2_altRequired setting to require the ALT attribute on images. But Drupal 8 always required that already anyway, so it doesn't affect us. Except that since the drupalimage plugin extends the image2 plugin, it's possible that we may need to make some small adjustments.

See also:

#1858210: [meta] Content editing experience follow-ups — in-place editing and WYSIWYG
#1950098: Update CKEditor library to 4.1
#2036253: Update CKEditor library to 4.2
#2039163: Update CKEditor library to 4.4
#2271051: Update CKEditor library to 4.4.4
#2345961: Update CKEditor library to 4.4.5
#2384581: Security: Update CKEditor library to 4.4.6
#2415111: Update CKEditor library to 4.4.7
#2521820: Update CKEditor library to 4.5.3
#2321583: Update CKEditor library to 4.5.5
#2663566: Update CKEditor library to 4.5.7
#2698587: Update CKEditor library to 4.5.8
#2724225: Update CKEditor library to 4.5.9
#2765751: Update CKEditor library to 4.5.10
#2797427: Update CKEditor library to 4.5.11