Test enhancement

[peter279k]

Changed log

• Using the class-based PHPUnit namespace to be compatible with the stable PHPUnit versions.
• Add the php-nightly test in Travis CI build and it allows that can allow failure.
• Set the different PHPUnit version to support the multiple PHP versions.
• Using the annotation expectedException to be compatible with the different PHPUnit versions.
• Separate the scheduleProvider and add the new test is for invalidScheduleProvider.
• Using the assertInternalType to assert the result type.

[codecov-io]

Codecov Report

Merging #9 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             master     #9   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       58     58           
=======================================
  Files             3      3           
  Lines           108    108           
=======================================
  Hits            108    108

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[adhocore]

Much thanks 👍 💯 🥇