Explicitly set `db_dir` in Rakefile
Explicitly set `db_dir` in Rakefile

Fixes the following error introduced in
81a60ac44197a021aa8a7b9876acb3b6642436d8.

    rake aborted!
    NameError: uninitialized constant ActiveRecord::Tasks::DatabaseTasks::Rails
    /home/koronen/.gem/ruby/2.3.0/gems/activerecord-4.2.6/lib/active_record/tasks/database_tasks.rb:55:in `db_dir'
    /home/koronen/.gem/ruby/2.3.0/gems/sinatra-activerecord-1.7.0/lib/sinatra/activerecord/rake.rb:112:in `db_dir'

We set `db_dir` explicitly to prevent `ActiveRecord` from using a default value
that references `Rails.application.config`.

Ref: <https://github.com/swanson/stringer/pull/428>

Fixes #436 and fixes #437.

geemus/stringer,jqr/stringer,jqr/stringer,Koronen/stringer,nibble/stringer,swanson/stringer,geemus/stringer,jqr/stringer,swanson/stringer,Koronen/stringer,dperson/stringer,geemus/stringer,nibble/stringer,dperson/stringer,swanson/stringer,jeroenj/stringer,jeroenj/stringer,geemus/stringer,Koronen/stringer,jqr/stringer,dperson/stringer,Koronen/stringer,nibble/stringer,jeroenj/stringer,jeroenj/stringer,dperson/stringer,swanson/stringer,nibble/stringer