Remove a warning to load mocha

For this message.

    *** Mocha deprecation warning: Change `require 'mocha'` to `require 'mocha/setup'`.
Specify to use mocha gem in test

Because test-unit gem is already specified.
Specify to use some gems for test by gemspec
Merge pull request #72 from kachick/improve/around_test_env

Improve around test dependencies