Amend placeholder file extension

Fixes #534
Remove unneeded code from the MySQL example

The `boxen?` method isn't used anywhere in the file. Remove it so that there's less code for people to copy and paste and to generate less confusion as to why there's a method there that doesn't need to be there.
Merge pull request #535 from mattr-/patch-1

Remove unneeded code from the MySQL example
Merge branch 'master' of https://github.com/boxen/our-boxen

Conflicts: