Remove sourcemap link from boot script

The replacement of var placeholders with values invalidates the
sourcemap locations and the sourcemap URL is invalid after the file is
copied from build/scripts/ to build/. Since the script is tiny the debug
info is not that valuable so this commit just removes the sourcemap URL
to avoid browsers complaining that they cannot retrieve it.

gulpfile.js

@@ -284,6 +284,9 @@ function generateBootScript(manifest) {
     .pipe(replace('__MANIFEST__', JSON.stringify(manifest)))
     .pipe(replace('__ASSET_ROOT__', defaultAssetRoot))
     .pipe(replace('__SIDEBAR_APP_URL__', defaultSidebarAppUrl))
+    // Strip sourcemap link. It will have been invalidated by the previous
+    // replacements and the bundle is so small that it isn't really valuable.
+    .pipe(replace(/.*sourceMappingURL.*/,''))
     .pipe(rename('boot.js'))
     .pipe(gulp.dest('build/'));
 }