Explicitly set email length for compat with Django 1.7 *and* 1.8

[frankier]

Django 1.8 increases default max_length of EmailField from 75 to 254. Since max_length is not currently specified, but the initial migration contains the 1.8 default of 254, Django 1.7 will think there's a migration to create, decreasing the max_length to 75. This can be fixed by explicitly setting the max_length to 254.

[spookylukey]

Thanks!