Add Ruby 2.1.1 to Travis build

In the process, allow failures from Ruby 2.1.0 due to:

    https://github.com/travis-ci/travis-ci/issues/2220

Change-Id: I4bbe1d4073f2a5fa92c6d1ff6cec1889a7ef96ab
Reviewed-on: http://gerrit.causes.com/37741
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Teach scss-lint --show-formatters

Show the installed formatters conveniently on the commandline.

    $ scss-lint --show-formatters
    Installed formatters:
     - Config
     - Default
     - Files
     - XML

Change-Id: Ic5aee057b7eea2ec847663b5d4e4c190e790a665
Reviewed-on: http://gerrit.causes.com/37742
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Update changelog with --show-formatters switch

Change-Id: I402ccae58c35e4c84b3a2901129c600f6f3f3af5
Reviewed-on: http://gerrit.causes.com/37743
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>
Add `--show-formatters` flag to README

Change-Id: I4210d1cf0ac2342b4b590152ac42f57d9c660d66
Reviewed-on: http://gerrit.causes.com/37745
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Adapt code for upcoming Rubocop

Fix some cops that will be active in the next version of Rubocop.

Change-Id: Iafd91b380ceb44e8c251e4a8e8333c0b1098083a
Reviewed-on: http://gerrit.causes.com/37816
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Add HexLength, HexNotation and `HexValidation`