Gemfile: pin i18n gem version 0.6.0

Gemfile: pin i18n gem version 0.6.0

Tests on CI server fails.
http://www.redmine.org/builds/logs/build_trunk_sqlite3_ruby-1.9.3-p194_580.html

<pre>
Failure:
  <"Due date changed from 01 01 2010 to 31 01 2010"> expected to be =~
  </01\/31\/2010/>.
test: IssuesHelper#show_detail with a due_date attribute should format the current date. (IssuesHelperTest)
test/unit/helpers/issues_helper_test.rb:127:in `block (3 levels) in <class:IssuesHelperTest>'
</pre>

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10247 e93f8b46-1217-0410-a6f0-8f06a7374b81