Change from Trusty to Xenial for Travis
Change from Trusty to Xenial for Travis

This is necessary, because we need full C++11 functionality,
and this requires GCC5+. Trusty has only GCC4.8.

theMarix/Gridcoin-Research,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,caraka/gridcoinresearch,caraka/gridcoinresearch,theMarix/Gridcoin-Research,caraka/gridcoinresearch,gridcoin/Gridcoin-Research,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,caraka/gridcoinresearch,caraka/gridcoinresearch,theMarix/Gridcoin-Research,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,theMarix/Gridcoin-Research,theMarix/Gridcoin-Research,Git-Jiro/Gridcoin-Research,theMarix/Gridcoin-Research,gridcoin/Gridcoin-Research,Git-Jiro/Gridcoin-Research,gridcoin/Gridcoin-Research,caraka/gridcoinresearch