Bump postcss and cssnano related dependencies

no issue
- an earlier dependency update resulted in production builds failing due to `cssnano`'s `postcss-merge-idents` dependency failing with a `Maximum call stack size exceeded` error
- bump `cssnano` to the 4.0.0 RC which includes a fix for this issue
- bump other `postcss` dependencies for good measure

ghost/admin/package.json

@@ -40,7 +40,7 @@
     "codemirror": "5.28.0",
     "coveralls": "2.13.1",
     "csscomb": "4.2.0",
-    "cssnano": "3.10.0",
+    "cssnano": "4.0.0-rc.2",
     "ember-ajax": "2.5.6",
     "ember-browserify": "1.2.0",
     "ember-cli": "2.14.2",
@@ -58,7 +58,7 @@
     "ember-cli-mocha": "0.14.4",
     "ember-cli-moment-shim": "3.1.0",
     "ember-cli-node-assets": "0.2.2",
-    "ember-cli-postcss": "3.5.0",
+    "ember-cli-postcss": "3.5.1",
     "ember-cli-pretender": "1.0.1",
     "ember-cli-selectize": "0.5.12",
     "ember-cli-shims": "1.1.0",
@@ -112,8 +112,8 @@
     "markdown-it-named-headers": "0.0.4",
     "matchdep": "1.0.1",
     "password-generator": "2.1.0",
-    "postcss-color-function": "3.0.0",
-    "postcss-custom-properties": "5.0.2",
+    "postcss-color-function": "4.0.0",
+    "postcss-custom-properties": "6.1.0",
     "postcss-easy-import": "2.1.0",
     "simplemde": "https://github.com/kevinansfield/simplemde-markdown-editor.git#ghost",
     "top-gh-contribs": "2.0.4",