fix error with variable as file name

Recently merged PR #1495 conflicted with PR #1490, this conflict was incorrectly "resolved" by me when rebasing, resulting in failures on travis-CI, as [indicated](https://github.com/mojombo/jekyll/pull/1495#issuecomment-25713419) by @sferik.

Problem was, #1490 went and validated the file name when the tag was created. However, it failed for variable syntax. Now, this PR fixes the validation to be performed after the variable was resolved.