Removes gulp-pleeease dependency

This gulp plugin was more trouble than it was worth.

ref https://github.com/roots/sage/issues/1386
ref https://discourse.roots.io/t/sourcemaps-no-longer-working-as-expected-with-sage-update/3022/1

CHANGELOG.md

@@ -1,6 +1,7 @@
 ### HEAD
 * Update to Bootstrap 3.3.4
 * Fix `gulp --production` race condition ([#1398](https://github.com/roots/sage/issues/1398))
+* Remove pleeease dependency in favor of vanilla gulp-autoprefixer and gulp-minify-css ([#1402](https://github.com/roots/sage/issues/1402))
 
 ### 8.1.0: March 13th, 2015
 * Move HTML5 Boilerplate's Google Analytics snippet to Soil ([#1382](https://github.com/roots/sage/issues/1382))