update redux form to version 6

[radziksh]

Hi, great starter! :) but I would like to note that there is already redux-form 6, and this version is not compatible with version 5, which is used in the starter because of fundamental design changes: http://redux-form.com/6.2.1/docs/MigrationGuide.md.

[rbrander]

The current react starter doesn't provide much in the way of forms. I see some code in the reducers, and the package.json includes v5.3.2.

@SethDavenport I suggest we remove the references to redux-forms as there is no example code using it.