add support for 7.4 branch

Author: spalger

docs/configuration.asciidoc

@@ -61,6 +61,7 @@ Options in node :::
   * `'6.8'`
   * `'5.6'`
   * `'7.3'` (unstable)
+  * `'7.4'` (unstable)
   * `'7.x'` (unstable)
   * `'master'` (unstable)
 
@@ -71,6 +72,7 @@ Options in the browser :::
   * `'6.8'`
   * `'5.6'`
   * `'7.3'` (unstable)
+  * `'7.4'` (unstable)
   * `'7.x'` (unstable)
   * `'master'` (unstable)
 

package.json

@@ -39,6 +39,7 @@
     ],
     "unstable_es_branches": [
       "7.3",
+      "7.4",
       "7.x",
       "master"
     ]