This hasn't been enabled in a while due to gRPC limitations

- Add `fast_finish` to travis config
- Specify that Go 1.9 is supported
- Additional error checking which was previously missed
- Make the migrations test better
-

In retrospect a primary key that has no relation to the certificate is preferred to using the certificate KeyID. The KeyID is also very large for a primary index.
This is a moderately tricky migration, especially for SQLite which has no means of altering the table in this fashion - it involves creating the new table and copying the data.
Order of commands also matters - index names are global in SQLite, so the `idx_expires_at` index needs to be created at the correct stages.

For MySQL migration the necessary steps are run as a single alter statement to minimise the risk of leaving the migration in an incomplete state if anything aborts.

When tested on a table with 250,000 rows (MySQL 5.7) the migration took 3 seconds to complete. As certificates will be requested infrequently the risk of prolonged locking is minimal.

Static content generator finds the root of the git repo and executes from there. Packr will scan subdirs for boxes to be packed.

Return an error if the store isn't known, instead of defaulting to a mem store