Build: Add SCSS-Lint to Travis build

Only run on CI to avoid adding back the Ruby dependency
SCSSLint: Use double quotes

Removed “%” for greyscale since units are unnecessary
SCSSLint: Fix spacing
SCSS: Fix SpaceBetweenParens
SCSSLint: Fix UrlQuotes

All URL references should use quotes
SCSSLint: SpaceBetweenParens

Remove unnecessary spaces inside parens
SCSSLint: Fix ZeroUnit

Units are unnecessary for zero values
SCSSLint: Fix SpaceAfterPropertyColon

Ensure space after property colon
SCSSLint: Fix SpaceAfterComma

Ensure spaces after each comma value
SCSSLint: Fix HexFormat

Use lower case and short forms for all hex colours
SCSSLint: Fix Shorthand

Use property shorthand values when possible
SCSSLint: Fix DuplicateProperty

This cannot be fully removed because of the RGB fallbacks
SCSSLint: Fix SpaceBeforeBrace

Ensure a space before all braces
SCSSLint: Fix TrailingSemicolonAfterPropertyValue

Add missing semicolons to all property values
SCSSLint: Fix CapitalizationInSelector

Some third party class naming must be excluded
SCSSLint: Fix ColorKeyword

Use hex colours instead of named colours
SCSSLint: Apply SingleLinePerSelector

Non-nested selectors are put on their own line. This is helpful for
line length and seeing all the roots of the selector
SCSSLint: Fix EmptyLineBetweenBlocks

This is a readability rule
SCSSLint: Fix BorderZero

Rather than "none", zero can be used for a shorter output
SCSSLint: Fix DeclarationOrder

Put `@extend` declarations at the top of rules