Update css processors

package.json

@@ -43,9 +43,9 @@
     "bugs": "https://github.com/TryGhost/Casper/issues",
     "contributors": "https://github.com/TryGhost/Casper/graphs/contributors",
     "devDependencies": {
-        "autoprefixer": "6.7.7",
+        "autoprefixer": "9.4.10",
         "beeper": "1.1.1",
-        "cssnano": "3.10.0",
+        "cssnano": "4.1.10",
         "gscan": "2.2.1",
         "gulp": "3.9.1",
         "gulp-filter": "5.1.0",
@@ -55,9 +55,9 @@
         "gulp-uglify": "3.0.2",
         "gulp-watch": "4.3.11",
         "gulp-zip": "4.2.0",
-        "postcss-color-function": "2.0.1",
-        "postcss-custom-properties": "5.0.2",
-        "postcss-easy-import": "1.0.1",
+        "postcss-color-function": "4.0.1",
+        "postcss-custom-properties": "8.0.9",
+        "postcss-easy-import": "3.0.0",
         "pump": "3.0.0"
     },
     "config": {