Check that db/ exists before running db:schema:load

[olivierlacan]

Note: I promise I'm not trying to DDoS you with Pull Requests. :-)

This is essentially the same issue as #156. An easy fix and useful
feedback for users who created a Rails app without ActiveRecord and
will not have any db/ directory.

The feedback is:

$ bin/rails db:schema:dump
The db/ directory doesn't exist, please create it.

Compare to what happened before this commit:

$ bin/rails db:schema:dump
rails aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - .../sandbox/appname/db/schema.rb
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `initialize'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `open'
.../oss/sequel-rails/lib/sequel_rails/railties/database.rake:26:in `block (3 levels) in <top (required)>'
.../sandbox/appname/bin/rails:9:in `require'
.../sandbox/appname/bin/rails:9:in `<top (required)>'
.../sandbox/appname/bin/spring:15:in `<top (required)>'
bin/rails:3:in `load'
bin/rails:3:in `<main>'
Tasks: TOP => db:schema:dump
(See full trace by running task with --trace)

[JonathanTron]

@olivierlacan Don't hesitate to DDoS the PR with your great contributions. Thanks again.