fix(valid-install): fix error message when CLI is run inside a ghost clone

no issue
- gulp has been replaced back with grunt, update error messaging

Author: acburdine

lib/command.js

@@ -19,9 +19,9 @@ const debug = createDebug('ghost-cli:command');
 function checkValidInstall(name) {
     if (fs.existsSync(path.join(process.cwd(), 'package.json')) &&
         fs.readJsonSync(path.join(process.cwd(), 'package.json')).name === 'ghost' &&
-        fs.existsSync(path.join(process.cwd(), 'gulpfile.js'))) {
+        fs.existsSync(path.join(process.cwd(), 'Gruntfile.js'))) {
         console.error('Ghost-CLI commands do not work inside of a clone or direct download.\n' +
-        `Perhaps you meant 'gulp ${name}'?`);
+        `Perhaps you meant 'grunt ${name}'?`);
         process.exit(1);
     }