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
92e2d6b777

Gemfile

@@ -46,7 +46,7 @@ gem 'rails_autolink'
 gem "will_paginate", '~> 3.0'
 gem "acts_as_list", "~> 0.2.0"
 
-gem "awesome_nested_set", ">= 3.0.0.rc.6"
+gem "awesome_nested_set", "3.0.0"
 
 gem 'color-tools', '~> 1.3.0', :require => 'color'