travis: Update to Go 1.11 from Go 1.10
travis: Update to Go 1.11 from Go 1.10

This unblocks @urvil38 who is working on a OPA-IPTables integration
for GSoC 2019.

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>

tsandall/contrib,tsandall/contrib,open-policy-agent/contrib,open-policy-agent/contrib,tsandall/contrib,open-policy-agent/contrib,tsandall/contrib,tsandall/contrib,open-policy-agent/contrib,tsandall/contrib,open-policy-agent/contrib,open-policy-agent/contrib,open-policy-agent/contrib,open-policy-agent/contrib,tsandall/contrib