fix(build): switch to cjs output for es5.

System output does not work at the current versions of TS and
system.js. Will revisit after upgrading TS.

Removes unused traceur tooling.

Closes #3974

tools/build/transpile.js

@@ -1,16 +0,0 @@
-var util = require('./util');
-var gulpTraceur = require('../transpiler/gulp-traceur');
-var file2moduleName = require('./file2modulename');
-
-module.exports = function(gulp, plugins, config) {
-  return function() {
-    return gulp.src(config.src)
-      .pipe(plugins.rename({extname: '.'+config.outputExt}))
-      .pipe(util.insertSrcFolder(plugins, config.srcFolderInsertion))
-      .pipe(gulpTraceur(
-        config.options,
-        file2moduleName)
-      )
-      .pipe(gulp.dest(config.dest));
-  };
-};