Reduce ApiKey key length to 128 (closes #757)

This avoids issues with MySQL where multi-byte encodings will cause a
256-character field to exceed the maximum index length (767 bytes).

https://github.com/toastdriven/django-tastypie/issues/757
Merge pull request #757 from acdha/757-shrink-apikey-db-field-length

Reduce ApiKey.key length to avoid MySQL's famous error: 'Failed to install index for tastypie.ApiKey model: Specified key was too long; max key length is 767 bytes'