Install mysql

[ealter]

This fixes #6

Before this change:

~/migrations/eliot$ docker run --volume $(pwd):/src --rm docteurklein/sqitch:mysql deploy -t eliot_test
Deploying changes to eliot_test                                                                                                                                      
  + tiny_table ............ not ok          
"mysql" failed to start: "No such file or directory"                                                                                                                 
                                         
Deploy failed

After this change (using sqitch:mysql as the tag for this docker container):

~/migrations/eliot$ docker run --volume $(pwd):/src --rm sqitch:mysql deploy -t eliot_test                                            
Deploying changes to eliot_test          
  + tiny_table .. ok

I also made a note in the readme about mounting the current directory into the docker container so that sqitch can access your migration files.