.travis.yml: Drop BAZEL_IMG

[wking]

The last consumer was removed in c483f59 (#117).

[yifan-gu]

/lgtm

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[wking]

The e2e-aws errors were:

3 error(s) occurred:

* module.vpc.aws_route.to_nat_gw[5]: index 5 out of range for list aws_route_table.private_routes.*.id (max 5) in:

${aws_route_table.private_routes.*.id[count.index]}
* module.vpc.aws_route_table_association.worker_routing[5]: index 5 out of range for list aws_route_table.private_routes.*.id (max 5) in:

${aws_route_table.private_routes.*.id[count.index]}
* module.vpc.data.aws_route_table.worker[5]: data.aws_route_table.worker.5: Your query returned no results. Please change your search criteria and try again.

Those don't look like flakes to me, but I can't think of how they'd relate to this change.

/retest