Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

src/core/image.js

@@ -53,9 +53,8 @@ var PDFImage = (function PDFImageClosure() {
   function handleImageData(image, nativeDecoder) {
     if (nativeDecoder && nativeDecoder.canDecode(image)) {
       return nativeDecoder.decode(image);
-    } else {
-      return Promise.resolve(image);
     }
+    return Promise.resolve(image);
   }
 
   /**