Ensure eloquent stored events are retrieved in order of ID.

[adrianb93]

I have an app with a postgres database. Records don't return in order of insert for me so my apps tests were failing. This PR does two things:

• So that the aggregate has events applied in the correct order, the query will be ordered by id.
• Supports the pgsql driver for this test suite.

[adrianb93]

I am not familiar with Travis. Is there a way to run the test suite twice on different configurations?

[riasvdv]

Testing both mysql and pgsql will be a bit more involved

• We'll need to update the docker-compose file
• Add the database driver as an env variable in Travis (I've just updated the Travis config to make this easier)

I suggest a different PR for this, then we can go ahead and merge this one.