Use resolve.root over resolve.modulesDirectories

This is a performance optimization recommended in gatsbyjs#280

---
`gatsby-starter-blog`

Before:
```
Command being timed: "gatsby build"
User time (seconds): 25.29
System time (seconds): 2.47
Percent of CPU this job got: 106%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:26.17
```

After:
```
Command being timed: "gatsby build"
User time (seconds): 24.29
System time (seconds): 2.83
Percent of CPU this job got: 110%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:24.56
```

---
`kyleamathews/blog`

Before:
```
Command being timed: "gatsby build"
User time (seconds): 32.10
System time (seconds): 2.57
Percent of CPU this job got: 107%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:32.31
```

After:
```
Command being timed: "gatsby build"
User time (seconds): 29.84
System time (seconds): 2.81
Percent of CPU this job got: 110%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:29.59
```

lib/utils/webpack.config.js

@@ -154,11 +154,9 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
         '.toml',
         '.yaml',
       ],
-      modulesDirectories: [
+      root: [
         directory,
-        `${__dirname}/../isomorphic`,
-        `${directory}/node_modules`,
-        'node_modules',
+        path.resolve(__dirname, '..', 'isomorphic'),
       ],
     }
   }