README - added note about vagrant-berkshelf version

If you are using Vagrant 1.4.3 use vagrant-berkshelf --plugin-version 1.3.7
to avoid error: ".vagrant.d/gems/gems/vagrant-berkshelf-2.0.0/lib/berkshelf/vagrant/env.rb:16:in `initialize': undefined method `opts' for #<Vagrant::
UI::Colored:0x565e7c8>"