Point to v3.1.1 on homepage

[mtfishman]

@emstoudenmire, could you have the homepage point to v3.1.1? It has an important bugfix for the removeQNs(ITensor) function.

It also has a few other improvements, like that svd now doesn't truncate by default (which it was doing accidentally before) and Cutoff=0 now truncates singular values that are exactly zero.

Thanks,
Matt

[emstoudenmire]

Just updated, including the change log. Good updates!