Fix error handling of missing new_issue_url config in url_for_new_issue.

Found this while digging into test failures in the vagrant environment. gitlabhq/gitlab-vagrant-vm#40

This changes that error to

    1) IssuesHelper url_for_new_issue should return path to external tracker
         Failure/Error: url_for_new_issue.should match(ext_expected)
         NoMethodError:
           undefined method `gsub' for nil:NilClass
         # ./app/helpers/issues_helper.rb:64:in `url_for_new_issue'
         # ./spec/helpers/issues_helper_spec.rb:96:in `block (3 levels) in <top (required)>'

Which is much more helpful in figuring out that there's a config problem. :)