Build task for creating os packages creates target

The target directory is created by the archives task, but it is not
created by the ospackages task, so if you do not have a target
directory and try to skip archives, the build will fail.

tasks/build/os_packages.js

@@ -12,6 +12,8 @@ export default (grunt) => {
   const fpm = args => exec('fpm', args);
 
   grunt.registerTask('_build:osPackages', function () {
+    grunt.file.mkdir('target');
+
     config.get('platforms')
     .filter(({ name }) => /linux-x(86|64)$/.test(name))
     .map(({ name, buildDir }) => {