Update travis-ci configuration to test in PHP7.3/4

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.

Author: beryllium

.travis.yml

@@ -7,13 +7,16 @@ cache:
     - $HOME/.composer/
 
 # Run tests against all these PHP versions
+# TODO: When it becomes possible in TravisCI, switch 7.4snapshot to plain 7.4
 php:
   - 5.4
   - 5.5
   - 5.6
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
+  - 7.4snapshot
   - hhvm
 
 before_script: