Fixed install warning

```cpp
./src/unix/pty.cc: In function ‘v8::Handle<v8::Value> PtyFork(const v8::Arguments&)’:
../src/unix/pty.cc:185:34: warning: ignoring return value of ‘int chdir(const char*)’, declared with attribute warn_unused_result [-Wunused-result]
       if (strlen(cwd)) chdir(cwd);
```