Build: Pin SCSSLint version for Travis to 0.22.0

Prevent build failures due to new SCSSLint releases introducing new
options
Build: Add David-DM badge to readme

Adds visibility to outdated devDependencies.
Swapped Travis to the new SVG badge at the same time.
Merge pull request #5156 from nschonni/pin-scss-lint-version

Build: Pin scss lint version & update SVG badges