Adds `7.3` and `7.4snapshot` to `.travis.yml`.
The `snapshot` part appears to be required at the moment, but I expect in a few days or weeks it will be able to switch to plain `7.4`.
I noticed that there is a deprecation notice for a bunch of the curly brace string accessors in the latest Release, please let me know if I can be of help to solve that. This library is used within [Sculpin](https://sculpin.io/), which I help maintain.
