How is possible to configure TLSv1.1 protocol for SSL connection in PostgreSQL?
@BrianEfting was correct, you can specify the appropriate cipher suites to only allow TLSv1.2 which should fit your PCI-DSS 3.1 specification needs. 

Using a cipher list like this in the ssl_ciphers option in your postgresql.conf:

ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:DHE-DSS-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:DHE-RSA-AES128-SHA256:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!3DES:!MD5:!PSK

along with setting ssl_prefer_server_ciphers=true, should be sufficient to allow only TLSv1.2 connections. 

You can verify this using SSLyze which knows about the PostgreSQL protocol.

To test, I used the following command:

./sslyze.py --sslv2 --sslv3 --tlsv1 --tlsv1_1 --tlsv1_2 localhost:5432 --starttls=postgres --hide_rejected_ciphers

Which gave the output below under PostgreSQL 9.4 on Debian Wheezy showing that all cipher suites except for the TLSv1.2 ciphers specified were rejected, which should satisfy the requirements of PCI-DSS 3.1 by using TLSv1.1 or greater.

postgres@pgsqlsec4:~/sslyze$ ./sslyze.py --sslv2 --sslv3 --tlsv1 --tlsv1_1 --tlsv1_2 localhost:5432 --starttls=postgres --hide_rejected_ciphers



 AVAILABLE PLUGINS
 -----------------

  PluginCompression
  PluginHeartbleed
  PluginChromeSha1Deprecation
  PluginSessionRenegotiation
  PluginOpenSSLCipherSuites
  PluginSessionResumption
  PluginHSTS
  PluginCertInfo



 CHECKING HOST(S) AVAILABILITY
 -----------------------------

   localhost:5432                      =&gt; ::1:5432



 SCAN RESULTS FOR LOCALHOST:5432 - ::1:5432
 ------------------------------------------

  * SSLV2 Cipher Suites:
      Server rejected all cipher suites.

  * TLSV1_2 Cipher Suites:
      Preferred:                       
                 ECDHE-RSA-AES128-GCM-SHA256   ECDH-256 bits  128 bits                                         
      Accepted:                        
                 ECDHE-RSA-AES256-SHA384       ECDH-256 bits  256 bits                                         
                 ECDHE-RSA-AES256-GCM-SHA384   ECDH-256 bits  256 bits                                         
                 DHE-RSA-AES256-SHA256         DH-1024 bits   256 bits                                         
                 DHE-RSA-AES256-GCM-SHA384     DH-1024 bits   256 bits                                         
                 ECDHE-RSA-AES128-SHA256       ECDH-256 bits  128 bits                                         
                 ECDHE-RSA-AES128-GCM-SHA256   ECDH-256 bits  128 bits                                         
                 DHE-RSA-AES128-SHA256         DH-1024 bits   128 bits                                         
                 DHE-RSA-AES128-GCM-SHA256     DH-1024 bits   128 bits                                         

  * TLSV1_1 Cipher Suites:
      Server rejected all cipher suites.

  * TLSV1 Cipher Suites:
      Server rejected all cipher suites.

  * SSLV3 Cipher Suites:
      Server rejected all cipher suites.



 SCAN COMPLETED IN 0.73 S
 ------------------------
postgres@pgsqlsec4:~/sslyze$