Fix for Node 10.x path lib

Currently, running `blacksmith` in node 10.x throws a type error:

```shell
$ blacksmith
Rendering: /blog

path.js:360
        throw new TypeError('Arguments to path.join must be strings');
              ^
TypeError: Arguments to path.join must be strings
    at path.js:360:15
    at Array.filter (native)
    at Object.exports.join (path.js:358:36)
    at /node_modules/blacksmith/lib/blacksmith/site.js:518:26
    at Array.map (native)
    at /node_modules/blacksmith/lib/blacksmith/site.js:515:55
    at Array.reduce (native)
    at Site.targetFiles (/node_modules/blacksmith/lib/blacksmith/site.js:474:32)
    at /node_modules/blacksmith/lib/blacksmith/site.js:529:29
    at Array.reduce (native)
```