Travis single-element lists as strings

.travis.yml

---
language: python
services:
  - postgresql

before_install:
  - pip install --upgrade pip
install:
  - pip install -r requirements/dev.txt
script:
  - coverage run -m pytest

# Travis will treat the below as equivalent
---
language: python
services: postgresql

before_install: pip install --upgrade pip
install: pip install -r requirements/dev.txt
script: coverage run -m pytest