fix 'Cannot read property 'posthtml' of null (#1578)

fixes #1574

Author: ranfdev

src/transforms/posthtml.js

@@ -25,13 +25,12 @@ async function transform(asset) {
 }
 
 async function getConfig(asset) {
-  let config =
-    (await asset.getPackage()).posthtml ||
-    (await asset.getConfig([
-      '.posthtmlrc',
-      '.posthtmlrc.js',
-      'posthtml.config.js'
-    ]));
+  let config = await asset.getConfig(
+    ['.posthtmlrc', '.posthtmlrc.js', 'posthtml.config.js'],
+    {
+      packageKey: 'posthtml'
+    }
+  );
   if (!config && !asset.options.minify) {
     return;
   }