src: bump the version number

There was a bug in the libuv release tool where it didn't properly
update the proper version file.
2014.02.28, Version 0.11.21 (Unstable)

Changes since version 0.11.20:

* unix: fix uv_fs_write when using an empty buffer (Saúl Ibarra
  Corretgé)

* unix, windows: add assertion in uv_loop_delete (Saúl Ibarra Corretgé)
Now working on v0.11.22