Increase SWAPFILE_SIZE to 4GB in edx_sandbox.yml

An error occurs when bower is installing its dependencies in the
'insights' role.

Increasing SWAPFILE_SIZE solves this problem.

playbooks/edx_sandbox.yml

@@ -27,7 +27,7 @@
     COMMON_ENABLE_SPLUNKFORWARDER: False
   roles:
     - role: swapfile
-      SWAPFILE_SIZE: 2GB
+      SWAPFILE_SIZE: 4GB
     - role: nginx
       nginx_sites:
       - certs