Create the docs directory if it doesn't exist

[m5rk]

No description provided.

[m5rk]

This error seems unnecessary:

$ API=Manifestly::V1::Checklist rake ga:docs
rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - docs/grape-apiary.md
/Users/markmceahern/.gem/ruby/2.2.1/gems/grape-apiary-0.1.2/lib/grape-apiary/tasks/grape-apiary.rake:10:in `initialize'
/Users/markmceahern/.gem/ruby/2.2.1/gems/grape-apiary-0.1.2/lib/grape-apiary/tasks/grape-apiary.rake:10:in `open'
/Users/markmceahern/.gem/ruby/2.2.1/gems/grape-apiary-0.1.2/lib/grape-apiary/tasks/grape-apiary.rake:10:in `block (2 levels) in <top (required)>'
/Users/markmceahern/.gem/ruby/2.2.1/gems/airbrake-5.0.3/lib/airbrake/rake/task_ext.rb:19:in `execute'
Tasks: TOP => ga:docs
(See full trace by running task with --trace)