Standard citation updated

The standard citation when Wannier90 is used in publications has been updated. Please cite
An updated version of Wannier90: A tool for obtaining maximally-localised Wannier functions
AA Mostofi, JR Yates, G Pizzi, YS Lee, I Souza, D Vanderbilt, N Marzari
Comput. Phys. Commun. 185, 2309 (2014) [ONLINE JOURNAL]