Phpunit v6 requires php7 to run

Using an older version of phpunit to allow testing on php5, as phpunit
v6 requires php7

Ref #16

Added support for SwiftMailer ^6.0

::newInstance() method has been deprecated, along with
Swift_MailTransport.

The default mail transport is now sendmail when using swiftmailer

https://legalhackers.com/advisories/SwiftMailer-Exploit-Remote-Code-Exec
-CVE-2016-10074-Vuln.html

Similar implementation of the PR #23 submitted by @badyto

Reused the same tests on that PR

There is also a possible breaking change with the output passed to the
callback in the `then` method, when executing a script the output will
be now an array instead of a string.

This test ensures that if a job is delayed by e.g. a previous job, it
will run if it was due when the scheduler ran.

* Allow run() to be executed multiple times

The current code expects the run() to be called only once. This is
fine if you start the scheduler from cron each minute and re-initialize
it each time. If you want to manually run the scheduler, or maybe
have a lot of jobs you do not want to init each minute, it would
be useful to call run() multiple times in the lifetime of the
scheduler.

In this case the collected data of the last run should be reset.
the executedJobs, failedJobs and outputSchedule.

* Allow queued jobs to be removed

If the scheduler is used to do multiple runs then it would be useful
to reset all queued Jobs. Currently the only way to do this is by
re-creating the scheduler object. But if the object is injected in the
code then this is not practical..

* Fix StyleCI analysis failure

Comment format fixed.

* Allow run() to be executed multiple times

After discussion with @peppeocchi make the re-running manually
triggered. So added a resetRun() method which can be called before
each run(). Also adjusted the test for this.

* Allow run() to be executed multiple times

The isDue check on the jobs is not provided a datetime in the run()
method. This means they use the creation time of the job as the
'current' time to compare against.

Most likely this creation time is approx the time run() is called so
normally this is no problem. But if setting up the job schedule takes
more time then I would say, using the creation time as run time is
unexpected. And if you run run() multiple times then it will always
run the same jobs because the time never changes.

So now provide an explicit run time for all jobs, which is the same
for all jobs each run.

* Fix StyleCI analysis failure

Remove empty line

Currently when run() is called it uses the current time as the run
timestamp. However it would be useful to run the scheduler at a
specific time. So allow an optional param with a DateTime to run().

Reasons for running at a specific time other than now() could be:

- Catching up on 'missed' runs, eg. when the system was down.

- Making sure the scheduler runs at an intended timestamp eg. when
cron is started at midnight it could be slow and start the php process
only at 00:01 and thus missing all midnight schedules.

- Improve testability of the scheduler (fake the time)