Make the asset root URL relative to the root of the package

When serving the package's contents via an npm CDN such as unpkg.com, it
will be more convenient if the asset root URL provided by the service
refers to the root of the package rather than the root of the `build/`
dir.

src/boot/boot.js

@@ -21,7 +21,7 @@ function injectScript(doc, src) {
 
 function injectAssets(doc, config, assets) {
   assets.forEach(function (path) {
-    var url = config.assetRoot + config.manifest[path];
+    var url = config.assetRoot + 'build/' + config.manifest[path];
     if (url.match(/\.css/)) {
       injectStylesheet(doc, url);
     } else {

src/boot/test/boot-test.js

@@ -77,7 +77,7 @@ describe('bootstrap', function () {
         'styles/inject.1234.css',
         'styles/pdfjs-overrides.1234.css',
       ].map(function (url) {
-        return 'https://marginal.ly/client/' + url;
+        return 'https://marginal.ly/client/build/' + url;
       });
 
       assert.deepEqual(findAssets(iframe.contentDocument), expectedAssets);
@@ -131,7 +131,7 @@ describe('bootstrap', function () {
         'styles/icomoon.1234.css',
         'styles/katex.min.1234.css',
       ].map(function (url) {
-        return 'https://marginal.ly/client/' + url;
+        return 'https://marginal.ly/client/build/' + url;
       });
 
       assert.deepEqual(findAssets(iframe.contentDocument), expectedAssets);