Duplicate logs table migrations from travis-logs

While technically more correct, redoing the logs migration via
JRuby is flaky due to the use of RVM and reliance on networks.
Remove logs table setup logic
Update README [skip ci]

logs table migration is now duplicated, so skip the steps about
loading the logs table from travis-logs.
Move logs table migration to testing logic

Since the `logs` table migration should not happen in production,
move it to a place migration does not search, and add a testing
step to set up the table correction for the tests.
Move auxiliary AR migrations to spec/migrations
Merge pull request #378 from travis-ci/ha-feature-logs_migration

Duplicate logs table migration