Remove unused variable. Fixes #30

It removes below ruby's warning.

* warning: assigned but unused variable
Remove unused variable in test

It removes below ruby's warning.

* warning: assigned but unused variable
Precede loading "mocha/setup" before "mocha" in test

It removes below mocha's warning.

* Change `require 'mocha'` to `require 'mocha/setup'`.
Merge branch 'kachick/improve-remove_warnings', closing #30 and #36

* kachick/improve-remove_warnings:
  Precede loading "mocha/setup" before "mocha" in test
  Remove unused variable in test
  Remove unused variable. Fixes #30