Removed Maria from docs

[wubzz]

Relates to knex/knex#2480

[tgriesser]

I think we can leave MariaDB in some parts of the docs, just for google purposes - but instead take out the mariasql library in the install section and make a note that if you want to use a MariaDB instance, you can just use the mysql driver because that works just fine and is better maintained.

[wubzz]

Updated. Cheers!