Travis otherwise runs SHARD= SHARDS=
causing this line
docker.test.shard: $(foreach image,$(shell echo $(images) | python util/balancecontainers.py $(SHARDS) | awk 'NR%$(SHARDS)==$(SHARD)'),$(docker_test)$(image))

to error with

```
awk: cmd. line:1: NR%==
awk: cmd. line:1:     ^ syntax error
usage: balancecontainers.py [-h] num_shards
balancecontainers.py: error: too few arguments
awk: cmd. line:1: NR%==
awk: cmd. line:1:     ^ syntax error
usage: balancecontainers.py [-h] num_shards
```