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>

--HG--
branch : ../sandbox/rails-4.1
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/sandbox/rails-4.1%4013373