Contact form 7 changed classes in 3.9 version. Now this is how it goes.
add_action( 'wpcf7_before_send_mail', 'process_form' );
function process_form( $cf7 ) {
$submission = WPCF7_Submission::get_instance();
if ( $submission ) {
$posted_data = $submission->get_posted_data();
}
if( $cf7->id() == 20 ) {
$wpdb->insert('newsletter_signups',
array(
'name' => $posted_data['name'],
'email' => $posted_data['email']
)
);
}
}
Read more about change here.