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

package.json

@@ -27,11 +27,11 @@
     "npm": ">=2.1.5"
   },
   "devDependencies": {
-    "pleeease": "3.2.0",
     "asset-builder": "^1.0.2",
     "browser-sync": "^2.4.0",
     "del": "^1.1.1",
     "gulp": "^3.8.11",
+    "gulp-autoprefixer": "^2.1.0",
     "gulp-changed": "^1.2.1",
     "gulp-concat": "^2.5.2",
     "gulp-flatten": "0.0.4",
@@ -40,7 +40,7 @@
     "gulp-jshint": "^1.9.4",
     "gulp-less": "^3.0.2",
     "gulp-load-plugins": "^0.9.0",
-    "gulp-pleeease": "^1.2.0",
+    "gulp-minify-css": "^1.0.0",
     "gulp-plumber": "^1.0.0",
     "gulp-rename": "^1.2.0",
     "gulp-rev": "^3.0.1",