Use bindler in favor of vagrant-plugin-bundler

[bindler](https://github.com/fgrehm/bindler) is more complete and more actively being developed. 

It also has the added benefit that `vagrant plugin bundle` installs the plugins into the project-local `.vagrant` directory, i.e. there are no conflicts between projects which use different versions of vagrant plugins.

The only loss in functionality is that we can't use the rsync fix for the vagrant-aws plugin. This will hopefully be resolved soon if
 * either PR mitchellh/vagrant-aws#77 is merged and 0.2.3 released
 * or bindler supports git locations: fgrehm/bindler#4
 * last resort: if it's urgent fork vagrant-aws, apply PR and release under a different name (boo!)