[Add] enable travis cache for pip
[Add] enable travis cache for pip

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