Use default RSpec pattern

See
https://github.com/rspec/rspec-core/commit/667a4ca00f50fddb3fca7cdfcaf97633ba267b1f#commitcomment-7725799.

'spec/**/*_spec.rb' is compatible with default value. See
https://github.com/rspec/rspec-core/blob/v3.1.2/lib/rspec/core/rake_task.rb#L17.