Use resolve.root when resolving loaders

This is a performance optimization recommended in gatsbyjs#280

There are a lot less loaders than general modules, so the performance
gain is minimal, but every second counts.

---
`gatsby-starter-blog`

Before:
```
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
```

After:
```
Command being timed: "gatsby build"
User time (seconds): 23.78
System time (seconds): 2.80
Percent of CPU this job got: 110%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:24.05
```

---
`kyleamathews/blog`

Before:
```
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

After:
```
Command being timed: "gatsby build"
User time (seconds): 26.70
System time (seconds): 2.79
Percent of CPU this job got: 112%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:26.30
```

lib/utils/webpack.config.js

@@ -363,11 +363,12 @@ module.exports = (program, directory, stage, webpackPort = 1500, routes = []) =>
     devtool: devtool(),
     output: output(),
     resolveLoader: {
+      root: [
+        path.resolve(directory, 'loaders'),
+        path.resolve(__dirname, '..', 'loaders'),
+      ],
       modulesDirectories: [
-        `${directory}/node_modules`,
-        `${directory}/loaders`,
-        `${__dirname}/../../node_modules`,
-        `${__dirname}/../loaders`,
+        'node_modules',
       ],
     },
     plugins: plugins(),