Fixes #324 Use branch name from config when creating PR

src/backends/github/API.js

@@ -393,7 +393,7 @@ export default class API {
     return this.deleteRef("heads", branchName);
   }
 
-  createPR(title, head, base = "master") {
+  createPR(title, head, base = this.branch) {
     const body = "Automatically generated by Netlify CMS";
     return this.request(`${ this.repoURL }/pulls`, {
       method: "POST",
@@ -518,5 +518,4 @@ export default class API {
       body: JSON.stringify({ message, tree, parents }),
     });
   }
-
 }