Update Travis CI config and add PHPUnit to dev requirements
Upgrade to PSR-4 autoloading
Updated library to conform to revision e88d5cd1ad of vnd.error

See
https://github.com/blongden/vnd.error/blob/e88d5cd1ad467b653573471f0c859428bddaece8/README.md
Simplify the directory structure
Updating README for the new version of rhumsaa\vnderror