pin minitest version 4

<pre>
$ rake db:migrate RAILS_ENV=test

*** Mocha deprecation warning: Change `require 'mocha'` to `require 'mocha/setup'`.

rake aborted!
MiniTest v5.0.6 is out of date.
`gem install minitest` and add `gem 'minitest' to you test helper.
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/turn-0.9.6/lib/turn/minitest.rb:23:in `<top (required)>'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `block in require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:236:in `load_dependency'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/activesupport-3.2.13/lib/active_support/dependencies.rb:251:in `require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/gems/turn-0.9.6/lib/turn.rb:13:in `<top (required)>'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `block (2 levels) in require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `each'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `block in require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `each'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `require'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448@global/gems/bundler-1.3.5/lib/bundler.rb:132:in `require'
/REDMINE-1/git-workdir/redmine/config/application.rb:7:in `<top (required)>'
/REDMINE-1/git-workdir/redmine/Rakefile:4:in `require'
/REDMINE-1/git-workdir/redmine/Rakefile:4:in `<top (required)>'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/bin/ruby_noexec_wrapper:14:in `eval'
/home/xxxxxx/.rvm/gems/ruby-1.9.3-p448/bin/ruby_noexec_wrapper:14:in `<main>'
(See full trace by running task with --trace)
</pre>