fix(gateway): downgrade Vert.x version to 3.4.1 to avoid classpath error with the gravitee-policy-groovy
fix(gateway): downgrade Vert.x version to 3.4.1 to avoid classpath error with the gravitee-policy-groovy

This closes gravitee-io/issues#687

gravitee-io/gateway,gravitee-io/gravitee-gateway,gravitee-io/gateway,gravitee-io/gravitee-gateway