Mention Go v1.8+ is required
Remove Travis test with Go 1.7.
language:gogo:-"1.8"-"1.9"-"1.10"-mastermatrix:allow_failures:-go:masterfast_finish:truebranches:only:-masterinstall:-go get github.com/spf13/pflag-go get github.com/pkg/errors-go get github.com/jung-kurt/gofpdf