Use master branch for core repository

This will be changed upon release, but newt from master downloads
blinky master and thus all project should be from master branches
diff --git a/project.yml b/project.yml
index 9d486f0..2de4bce 100644
--- a/project.yml
+++ b/project.yml
@@ -27,6 +27,6 @@
 #
 repository.apache-mynewt-core:
     type: github
-    vers: 1.12.0
+    vers: 0.0.0
     user: apache
     repo: mynewt-core