Keep Ruby version up to date and watch out for sourcemaps

[jeroenptrs]

Hey there. I've been able to set up tests for Sass-only projects thanks to this article, so thanks!
Along the way, I've stumbled across a few things that spawned build fails:

If you're using the latest version of Sass, make sure Ruby is using the latest version as well. Travis doesn't provide the latest version (2.4.1) of Ruby on standard, which prompted a build fail. I updated my .travis.yml like this:

script:
- "test/travis.rb"

language: ruby
rvm:
- 2.4.1
before_install:
- gem install sass
- chmod +x test/travis.rb

Also Sass now adds sourcemaps on default when you compile to a compressed css. In case of those silent class checks, that you mentioned at the end of your article, this will spawn a build fail again.
This is because the file isn't empty anymore but contains the sourcemap file's name in comments. To counter this, you have to add --sourcemap=none to avoid Sass from generating a sourcemap file.

The test ultimately becomes:

result = `sass test/silent.scss build.silent.css -t compressed --sourcemap=none`
raise result unless $?.to_i == 0
raise "When $use-silent-classes is set to true the module should not output any CSS" unless File.size('build.silent.css') == 0
puts "Silent sass compiled successfully"

Anyway, thanks again for the handy article. Hope my contribution can help someone out there as well!