Use bindler in favor of vagrant-plugin-bundler

[tknerr]

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 Fix rsync on windows (alternative) mitchellh/vagrant-aws#77 is merged and 0.2.3 released
• or bindler supports git locations: Install from git locations fgrehm/bindler#4
• last resort: if it's urgent fork vagrant-aws, apply PR and release under a different name (boo!)