Ensure node version is satisfied

bin/probot

@@ -1,5 +1,13 @@
 #!/usr/bin/env node
 
+const semver = require('semver');
+const version = require('../package').engines.node;
+
+if (!semver.satisfies(process.version, version)) {
+  console.log(`Node.js version ${version} is required. You have ${process.version}.`);
+  process.exit(1);
+}
+
 require('commander')
   .version(require('../package').version)
   .usage('<command> [options]')