Update fields to work with unicode collation in mysqldb

### Problem 
When trying to install RedFlash I get the following error (full text at https://gist.github.com/4195594):

    _mysql_exceptions.Warning: Specified key was too long; max key length is 1000 bytes

A bit of Googling leads to the following explanation - http://stackoverflow.com/questions/4547479/error-during-djangos-syncdb-on-server

I can confirm that I am using a utf encoded DB, with the following conf (in `/etc/mysql/conf.d/my.cnf`):

    [mysqld]
    character-set-server    = utf8
    collation-server        = utf8_unicode_ci
    init_connect            = 'set collation_connection = utf8_unicode_ci;'


### Solution
I have updated the RedFlash Squawk migrations to work with UTF-8 encoded MySQL databases, by reducing the length of the `gateway_response` and `gateway_status` columns to 300 chars long, rather than 400.