Test enhancement

[peter279k]

Changed log

• Let the package require PHP >=5.6 version.
• Remove the additional space.
• Add the php-nightly test in Travis CI build and let this allow failure.
  We don't guarantee that the php-nightly test will always be successful.
• Set the different PHPUnit version to support multiple PHP versions.
• Using the class-based PHPUnit namespace to be compatible with the latest stable PHPUnit version.

[peter279k]

@stevleibelt, can you concern about this PR?

Thanks.

[stevleibelt]

Hey @peter279k,

thanks for your pull request and for your follow up.

Your pull request is perfect and I am totally ashamed of the whitespaces :-D.

[stevleibelt]

Now I need to add a contribution section 👍