Add tests to run on pull request

[stevector]

This repository would benefit from

• Automated code style checks.
• Tests of the cache behavior. These could likely be copied and adapted from another Drupal 7 module, perhaps memcache. Copy/modify is how we wrote tests for the wp-lcache and the Drupal 8 LCache module. These tests could run entirely inside Travis or Circle CI.
• Tests of any Pantheon-specific behavior.