Update mtime on scss target file

[gpeterso]

Modify the custom-scss task so that it updates the mtime on the target file (css/custom-scss-compiled.css).

Why is this needed? Normally, gulp.dest() will simply propagate the source file's mtime to the target file. In this case, the source file is scss/main.scss.

Suppose your main.scss file contains the following line:

@import "variables";

If you modify scss/_variables.scss, the watch-custom-scss task will trigger an update to css/custom-scss-compiled.css). However, the updated file's mtime will remain unchanged because the mtime on scss/main.scss has not changed. As a result, the watch-css task will not know that css/custom1.css needs to be rebuilt.

I'm guessing this is an unintended side-effect of changing the watch-css task to use polling instead of filesystem events (f34630e). Simply touching the target file seems like the most straightforward solution.