More permissive sed call on BSD hosts to prevent errors when cleaning nfs

Gnu sed -i option doesn't support a space between the option and the
backup extension.

On BSD hosts running GNU sed (for instance OSX with Macports), it cleaning
nfs shares couldn't happen.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>

lib/vagrant/hosts/bsd.rb

@@ -42,7 +42,7 @@ module Vagrant
           if $?.to_i == 0
             # Use sed to just strip out the block of code which was inserted
             # by Vagrant
-            system("sudo sed -e '/^# VAGRANT-BEGIN: #{env.vm.uuid}/,/^# VAGRANT-END: #{env.vm.uuid}/ d' -i bak /etc/exports")
+            system("sudo sed -e '/^# VAGRANT-BEGIN: #{env.vm.uuid}/,/^# VAGRANT-END: #{env.vm.uuid}/ d' -ibak /etc/exports")
           end
         end
       end