Improve security by making sure that we use a safe set of ciphers.
This comes from Debian bug #804196 (LP Bug #1517265). When Midori uses libwebkitgtk, it inherits the default value of the G_TLS_GNUTLS_PRIORITY environment variable. Unfortunately this value is not safe enough because it still lets GnuTLS to enable to unsafe ciphers. Therefore, we need to set this variable ourselves and make sure that Midori is using safe ciphers for TLS operations.