Bug#831960: bumping severity to serious

[Lucas Nussbaum]

severity 831918 serious
severity 831921 serious
severity 831933 serious
severity 831944 serious
severity 831945 serious
severity 831950 serious
severity 831960 serious
severity 831961 serious
severity 831963 serious
thanks

Actually, after discussion on #830997, those bugs should really be
severity:serious as they are about a missing build-indep target in
debian/rules, which has been mandatory for some time.

Lucas