Update haproxy to 1.5.3.

2014/07/25 : 1.5.3
- DOC: fix typo in Unix Socket commands
- BUG/MEDIUM: connection: fix memory corruption when building a proxy
  v2 header
- BUG/MEDIUM: ssl: Fix a memory leak in DHE key exchange
- DOC: mention that Squid correctly responds 400 to PPv2 header
- BUG/MINOR: http: base32+src should use the big endian version of
  base32
- BUG/MEDIUM: connection: fix proxy v2 header again!