Minor Bug Fixes

• Fix an issue where using a select query mod in a one-to-many relationship lookup was ignored (#159)
• Fix an issue where an enum with only one value could break the tests (#161, thanks @gsamokovarov)
• Fix an issue where using DeleteAll() after a relationship helper would break (#155)
• Fix another instance where eager loading would fail with conversion errors (#158)
• Disabled MSSQL tests in CI (mssql docker image no longer provides a version of mssql that works for sqlboiler)
• Syntax highlighting in README (thanks @Alexendoo)