set shared folder permission to 755 instead of 777 default

The to open shared folder permission cause failing mocha tests of the heroku-softcover 
project when they was performed inside the shared folder.

Solution found here https://coderwall.com/p/z1kjww but has to be adapted to the new vagrant version now the line looks like config.vm.synced_folder ".", "/vagrant", :mount_options => ["dmode=755","fmode=755"]