INTERNALS: require GnuTLS >= 2.11.3

Since the public pinning support was brought in e644866caf. GnuTLS
2.11.3 was released in October 2010.

Figured out in #2890

docs/INTERNALS.md

@@ -78,7 +78,7 @@ Dependencies
 ------------
 
  - OpenSSL      0.9.7
- - GnuTLS       1.2
+ - GnuTLS       2.11.3
  - zlib         1.1.4
  - libssh2      0.16
  - c-ares       1.6.0