FTBFS: undefined method `mock' for #<RSpec::ExampleGroups::YARDCLICommandParser::Run:..

Imported from Debian bug http://bugs.debian.org/794148:

Source: yard
Version: 0.8.7.6-1
Severity: serious
Tags: sid stretch
Justification: fails to build from source
User: <email address hidden>
Usertags: ftbfs

Dear Maintainer,

The package fails to build:

Failures:

  1) YARD::CLI::CommandParser#run should show help if --help is provided
     Failure/Error: command = mock(:command)
     NoMethodError:
       undefined method `mock' for #<RSpec::ExampleGroups::YARDCLICommandParser::Run:0x00000002d81448>
     # ./spec/cli/command_parser_spec.rb:10:in `block (3 levels) in <top (required)>'

....

  7) YARD::CLI::Config Viewing an item should view item if no value is given
     Failure/Error: YARD::Config.stub!(:save)
     NoMethodError:
       undefined method `stub!' for YARD::Config:Class
     # ./spec/cli/config_spec.rb:9:in `block (2 levels) in <top (required)>'

....

/usr/bin/ruby2.1 /usr/bin/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb failed
ERROR: Test "ruby2.1" failed. Exiting.

Full build log:
https://reproducible.debian.net/rb-pkg/unstable/amd64/yard.html

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.19.0-23-generic (SMP w/8 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)