Improve the integration tests

FluentMigrators' integration tests need to be improved. I'm starting a pull request to gather any ideas/problems.

Problems that exists today:
- A lot of tests are not repeatable if they fail (they don't cleanup properly after themselves)
- Most of them ignore the IntegrationTestOptions settings and run the tests for databases despite them not being enabled in the settings and not having a correct connection string.
- The MigrationRunnerTests class is huge and needs to be broken up.
- The integration tests should be run by Travis CI.

I'm starting off with one commit - a little cleanup of SqlServerSequenceTests.cs class. I'll be adding more over the next few days.