[Add] travis-ci: Added gcc-7 build
[Add] travis-ci: Added gcc-7 build

bigdig/vnpy,andrewchenshx/vnpy,vnpy/vnpy,andrewchenshx/vnpy,bigdig/vnpy,msincenselee/vnpy,bigdig/vnpy,vnpy/vnpy,msincenselee/vnpy,msincenselee/vnpy,msincenselee/vnpy,andrewchenshx/vnpy,bigdig/vnpy,andrewchenshx/vnpy,andrewchenshx/vnpy