Gemfile: pin awesome_nested_set version on 3.0.0

Test fails on awesome_nested_set 3.0.1.

<pre>
  1) Error:
IssueTest#test_destroying_a_deleted_issue_should_not_raise_an_error:
ActiveRecord::StaleObjectError: Attempted to destroy a stale object: Issue
    app/models/issue.rb:188:in `destroy'
    test/unit/issue_test.rb:1419:in `block (2 levels) in test_destroying_a_deleted_issue_should_not_raise_an_error'
    test/unit/issue_test.rb:1418:in `block in test_destroying_a_deleted_issue_should_not_raise_an_error'
    test/unit/issue_test.rb:1417:in `test_destroying_a_deleted_issue_should_not_raise_an_error'
</pre>

https://github.com/collectiveidea/awesome_nested_set/issues/280
https://github.com/marutosi/openproject/commit/92e2d6b7775c9a0d5901537921f9876125fb9bf1
Merge pull request #2121 from marutosi/finn/rails4/awesome_nested_set-pin-3.0.0

Gemfile: pin awesome_nested_set version on 3.0.0