undefined element when processing polymer invocations

Hi,

Trying to vulcanize and running into this error.

```
        var invocation = utils.processPolymerInvocation(elementName, match);
                                                        ^
ReferenceError: elementName is not defined
    at Object.<anonymous> (/Users/foo/git/vulcanize/lib/vulcan.js:230:57)
    at exports.each (/Users/foo/git/vulcanize/node_modules/whacko/lib/api/traversing.js:294:24)
    at Object.handleMainDocument [as processDocument] (/Users/foo/git/vulcanize/lib/vulcan.js:222:26)
    at /Users/foo/git/vulcanize/bin/vulcanize:109:10
    at /Users/foo/git/vulcanize/lib/vulcan.js:33:5
    at Object.processOptions (/Users/foo/git/vulcanize/lib/optparser.js:92:3)
    at Object.setOptions (/Users/foo/git/vulcanize/lib/vulcan.js:28:13)
    at Object.<anonymous> (/Users/foo/git/vulcanize/bin/vulcanize:104:8)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
```

Originated here: https://github.com/Polymer/vulcanize/commit/7d5256fa6dfbc276e3f9cfabdd9d483a4f5f3f3f#diff-2fd25663c1d3dca2153ce60dac0f7245R225

Thank you,

Jon