Update to Slick 2.1.0-RC1
Remove `SessionWithAsyncTransactionForTesting`

`slick.jdbc.JdbcBackend#BaseSession` has `isInTransaction` in Slick
2.1.0, so we don't need to add it by ourselves anymore.
Merge pull request #189 from guersam/tmp/slick-2.1.0-RC1

Update to Slick 2.1.0-RC1