When upgrading from 4.7.7 to 4.7.8 I get error "Specified key was too long"
This issue is CRM-18983: DB error on upgrade to 4.7.8. I ran into it on OSX also while testing an upgrade.

There is a PR at https://github.com/civicrm/civicrm-core/pull/8605 currently which addresses this by reducing the size of the key before increasing the column size. Not all sites will have the option of reconfiguring MySQL server settings to accommodate the DB change in 4.7.8, so it makes sense to me that we do this in the schema.

Applying the patch before running the upgrade works for me - please post feedback on the JIRA issue or PR if you find it helps.