Updates ffi to 1.9.6

- Updates listen gem ffi gem sub-dependency to 1.9.6 from 1.9.3. Spring
uses listen but does not currently support version 2.x.
- Adds code comment to gemfile noting why listen gem is present.
Updates quiet_assets to 1.0.3
Merge pull request #693 from codeforamerica/upgrade-quiet_assets

Updates quiet_assets to 1.0.3
Merge pull request #692 from codeforamerica/upgrade-listen

Updates ffi to 1.9.6
Updates coveralls to 0.7.1