Test support for MySQL and PostgreSQL

[snlamm]

Grindjs commits to supporting SQLite, MySQL, and PostgreSQL databases. This PR adds MySQL and PostgreSQL test support to help ensure grind-orm continues to run smoothly with all three services.

Features:

• Configurations for each database.
• Tests run once each for each database.
• Integrates with Travis CI.

Details:

• Adds mysql and pg packages.
• The README has instructions for configuring Mysql and Postgres for local testing.
• Tests can be limited to certain services (by default they use all three). Uses pattern in test names to determine which service to use.
• Added --serial flag to AVA setup so test files run serially instead of asynchronously. This ensures only one migration is running at a time and avoids lock issues.
• Reduced fake-ID sizes in some tests to get them within the Postgres integer size limit.
• Resolved a few linter issues across the app, mostly to do with adding missing blank lines.