[FIX] requirements: bump up recommended psycopg2 version

Version 2.7.1 is not yet available in most distributions,
but is now considered the recommended version.

Odoo will work just fine with any psycopg2 version >= 2.2, though.

requirements.txt

@@ -19,7 +19,7 @@ mock==1.0.1
 passlib==1.6.2
 psutil==2.1.1
 psycogreen==1.0
-psycopg2==2.5.3
+psycopg2==2.7.1
 pyPdf==1.13
 pydot==1.0.2
 pyparsing==1.5.7