Remove note about Playdoh from CONTRIBUTING.md

All open pull requests were submitted after the Playdoh upgrade.
Update nav chevron detection and mobile slider toggle CSS bug 878871
Cache-bust a few remaining style guide images
(Bug 881125)
Enable get phone Spain (es) Firefox OS bug 878871
Update nav js to activate adaptive chevron on #soccer-hook. Bug 878871.
Merge pull request #1031 from jpetto/bug-878871-update-nav-chevron

Update nav js to activate adaptive chevron on #soccer-hook. Bug 878871.
Merge pull request #1030 from alexgibson/firefox-os-minor-updates-bug-878871

Update nav chevron detection and mobile slider toggle CSS bug 878871
Merge pull request #1026 from sgarrity/bug-881125-style-guide-firefox-logo

Cache-bust a few remaining style guide images
Merge pull request #1032 from alexgibson/enable-get-phone-spain-firefox-os-bug-878871

Enable get phone Spain (es) Firefox OS bug 878871
Merge pull request #1025 from openjck/contributing-update

Remove note about Playdoh from CONTRIBUTING.md