improve version check.

lib/program.js

@@ -95,7 +95,7 @@ function Program(options) {
     if (!self.tmux) return 2;
     try {
       version = cp.execFileSync('tmux', ['-V'], { encoding: 'utf8' });
-      return +/([\d.]+)/.exec(version)[1];
+      return +/^tmux ([\d.]+)/i.exec(version.trim().split('\n')[0])[1];
     } catch (e) {
       return 2;
     }