vagrant: Use id -g for finding a user's group.

On OSX, the user id and group id don't match.  So while the previous
code was always wrong, it produced incorrect output there.  We can fix
this by replacing `whoami` with `id -g` for finding the current user's
group ID.

Vagrantfile

@@ -202,7 +202,7 @@ if [ ! -w /srv/zulip ]; then
     # sudo is required since our uid is not 1000
     echo '    vagrant halt -f'
     echo '    rm -rf /PATH/TO/ZULIP/CLONE/.vagrant'
-    echo '    sudo chown -R 1000:$(whoami) /PATH/TO/ZULIP/CLONE'
+    echo '    sudo chown -R 1000:$(id -g) /PATH/TO/ZULIP/CLONE'
     echo "Replace /PATH/TO/ZULIP/CLONE with the path to where zulip code is cloned."
     echo "You can resume setting up your vagrant environment by running:"
     echo "    vagrant up"

docs/development/setup-vagrant.md

@@ -889,7 +889,7 @@ It is okay to proceed and start the Zulip server.
 ##### Permissions errors
 
 When building the development environment using Vagrant and the LXC provider,
-if you encounter permissions errors, you may need to `chown -R 1000:$(whoami)
+if you encounter permissions errors, you may need to `chown -R 1000:$(id -g)
 /path/to/zulip` on the host before running `vagrant up` in order to ensure that
 the synced directory has the correct owner during provision. This issue will
 arise if you run `id username` on the host where `username` is the user running

docs/testing/testing.md

@@ -126,7 +126,7 @@ something valuable to helping keep Zulip bug-free.
 
 - When building the development environment using Vagrant and the LXC
   provider, if you encounter permissions errors, you may need to
-  `chown -R 1000:$(whoami) /path/to/zulip` on the host before running
+  `chown -R 1000:$(id -g) /path/to/zulip` on the host before running
   `vagrant up` in order to ensure that the synced directory has the
   correct owner during provision. This issue will arise if you run `id
   username` on the host where `username` is the user running Vagrant