prevent rake task error on JRuby 1.7 (#12228)

prevent rake task error on JRuby 1.7 (#12228)

Following is "rake db:migrate RAILS_ENV=test" output
on jruby 1.7.0 (1.9.3p203) 2012-10-22 ff1ebbe on OpenJDK Client VM 1.6.0_18-b18 [linux-i386].

<pre>
ArgumentError: file not found: db:migrate
   non_options at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:167
          map! at org/jruby/RubyArray.java:2387
   non_options at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:146
   non_options at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:207
  process_args at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:52
          _run at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/minitest/unit.rb:891
           run at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/minitest/unit.rb:884
           run at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:21
       autorun at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:326
      run_once at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:27
       autorun at /xxxxx/rubies/jruby-1.7.0/lib/ruby/1.9/test/unit.rb:325
</pre>

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10737 e93f8b46-1217-0410-a6f0-8f06a7374b81