Set default provider to "email" in migration

[colavitam]

This change should fix incompatibility with standard devise controllers using default
migrations. Standard devise allows for provider to be null, but devise_token_auth requires it to have a value (as enforced by the default migration). The generated migration now sets this field to "email" by default. It should resolve #257 when the new migration is used. Setting the provider in the standard registrations controller is now redundant, but I left it as is to avoid breaking existing installations using the old migration.

[colavitam]

It may also be important to note that the scheme this app uses for setting the provider column isn't consistent with that used by standard devise. Changing the controllers and migrations to allow provider to be nil instead of "email" may make more sense, but it would likely make supporting users created by earlier versions more difficult.

[booleanbetrayal]

This seems sane to me. Thanks for the PR @colavitam !