Improve travis config
Improve travis config

* `pipenv install` with `--dev`.
    * We don't need these tools now, but will when we lint on Travis.
    * Also, this validates that the dev install is working.
* Cache pipfiles. (From travis file for requests, which uses pipenv —
  and is by its author.)
* Canonical indentation for `-` lines

olinlibrary/ABE,olinlibrary/ABE,olinlibrary/ABE