botocore incompatible message upon installation

Didn't see after I searched though I didn't look _that_ far into it so hopefully this isn't a dupe.

```terminal
Successfully built s4cmd
awscli 1.11.56 has requirement botocore==1.5.19, but you'll have botocore 1.12.19 which is incompatible.
Installing collected packages: urllib3, botocore, boto3, pytz, s4cmd
Found existing installation: botocore 1.5.19
Uninstalling botocore-1.5.19:
Successfully uninstalled botocore-1.5.19
Successfully installed boto3-1.9.19 botocore-1.12.19 pytz-2018.5 s4cmd-2.1.0 urllib3-1.23
```

env: pip 18, python 2.7, macOS 10.something

Reproduce the reported installation with pip 18, Python 2.7, and macOS, using the versions shown for s4cmd, boto3, and botocore. Start by checking the project's dependency declarations; done means the installation no longer reports an incompatible botocore requirement or the cause is clearly documented.

Written by the indexing model from the issue text.