Add xenial to matrix of Travis builds
Add xenial to matrix of Travis builds

With the current setup, the Trusty+1.11 build breaks, so it's excluded.

Maxwell-Alexius/goby,goby-lang/goby,Maxwell-Alexius/goby,Maxwell-Alexius/goby,st0012/Rooby,goby-lang/goby,st0012/Rooby,goby-lang/goby