Use gulp-concat-css to bubble up @import statements

[jeroennoten]

Using gulp-concat-css instead of gulp-concat makes sure that @import statements are bubbled up to the top of the file. This is necessary because @import statements that are not at the top of the file are ignored by the browser (as per the standard). The options {rebaseUrls: false, inlineImports: false} makes this change non-breaking.

I discovered this issue when I was concatenating vendor css files where one of them had an @import statement to import a Google Font:

@import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,600,700,300italic,400italic,600italic);

This ended up somewhere in the middle of the output file and was ignored by the browser. gulp-concat-css takes care of this problem.

[JeffreyWay]

This PR messed up CSS sourcemaps so I reverted it.

748d2af

You can resubmit if you fix that.

[jeroennoten]

Okay sorry for the issues. I will look into a solution.