Fix Sass output style option

Problem/Motivation

The option passed to gulp-sass specifies style: 'compressed', however the actual output is not compressed.

This is because the key for the option should be outputStyle, not style (reference).

To repeat

Create a sub theme and run gulp.

Expected

Per the Sass docs, the output should

[have] no whitespace except that necessary to separate selectors and a newline at the end of the file. It also includes some other minor compressions, such as choosing the smallest representation for colors. It’s not meant to be human-readable

Actual

The output has linebreaks and other whitespace that it shouldn't, and it is very human-readable.

Proposed resolution

Change the option key from style to outputStyle.

In limited testing with a preexisting sub theme, this change achieves the expected result.