gui: Downgrade Angular 1.5.3 -> 1.2.9 (fixes #2961)

I haven't been able to figure out the problem, despite a lot of
experimenting with this...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2988

gui/default/vendor/angular/README.md

@@ -1,6 +1,6 @@
 The files contained herein are:
 
- - angular 1.5.3
+ - angular 1.2.9
  - angular-translate 2.9.0.1
  - angular-translate-loader-static-files 2.11.0
  - angular-dirPagination 759009c