ci: force previous version of travis distribution (#2193)
ci: force previous version of travis distribution (#2193)

Fixes ipv6 errors with the latest Travis Trusty image from 2017-12-12.
envoyproxy/envoy,istio/envoy,dnoe/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,istio/envoy,jrajahalme/envoy,craffert0/envoy,envoyproxy/envoy,eklitzke/envoy,lizan/envoy,sebrandon1/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,lyft/envoy,lizan/envoy,eklitzke/envoy,eklitzke/envoy,dnoe/envoy,lizan/envoy,dnoe/envoy,lizan/envoy,jrajahalme/envoy,envoyproxy/envoy,jrajahalme/envoy,lyft/envoy,eklitzke/envoy,jrajahalme/envoy,istio/envoy,lizan/envoy,jrajahalme/envoy,dnoe/envoy,craffert0/envoy,envoyproxy/envoy,sebrandon1/envoy,istio/envoy,envoyproxy/envoy,istio/envoy,eklitzke/envoy,lyft/envoy,istio/envoy,lizan/envoy,envoyproxy/envoy,sebrandon1/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,dnoe/envoy,envoyproxy/envoy,sebrandon1/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,sebrandon1/envoy,jrajahalme/envoy,eklitzke/envoy,craffert0/envoy,craffert0/envoy