Adds cred_counter to Hosts.

[Feature #48595211]

* Migration to add column to hosts
* Manual counter column due to issues with association of host and cred through service
Bumped version.
Adding line break below private for clarity.
Merge pull request #15 from rapid7/creds-cache-counter

[Delivers #48595211] - Adds cred_counter to Hosts
Bumped Version
Merge branch 'master' into host_factories
updating the dummy schema