upgrade canvas-rce to v2.5.6
upgrade canvas-rce to v2.5.6

fixes issue with inline file preview for document links inserted from
the rce sidebar

refs CORE-378

test plan:
- have canvadocs plugin configured
- make sure rcs feature flag is enabled
- create/edit a wiki page
- upload/insert a pdf
- save
- the inline preview should work
- edit the page
- put the cursor on the link
- click the link button in the editor toolbar
- test wil both checkboxes (autoexpand/hide inline preview)

Change-Id: I1fd5d8a41ae34f325a63b98d86b0d98ab3e2f34f
Reviewed-on: https://gerrit.instructure.com/131752
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Tested-by: Jenkins
QA-Review: Collin Parrish <aa381382ccdcc876f5cf7879d921ed03200f6804@instructure.com>
Product-Review: Brent Burgoyne <bccf16d7abdd2611580b1cd7d7a6fcc150d619da@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms