Remove php 5.6

[phil-davis]

In discussion, it has been pointed out that there are more places to remove mention of PHP 5.6:

1. root composer.json of the app - bump platform php to 7.0.8 and then do composer update to get an updated composer.lock file.

2. vendor-bin/behat/composer.json - bump platform php to 7.0.8 - composer.lock is not configured, so no need to touch that, it is auto-generated when tests start. This will only be in apps that have Behat acceptance tests

3. bump the ownCloud server min-version in appinfo/info.xml to 10.2 - because actually we are running automated testing against the core stable10 QA tarball, which will next become 10.2. So we are not automatically sure that the app still works with core releases 10.0.* or 10.1.* (e.g. maybe some app code gets added/changed and the developer uses PHP 7.0-only syntax that does not work on PHP 5.6)

Points (1) and (2) seem straightforward and "have to happen"

Point (3) could be avoided if we have a solution.process for owncloud/QA#611
which ensures that apps are tested against any older supported server versions prior to app release (and maybe every night in the automated testing)

[codecov]

Codecov Report

Merging #718 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #718   +/-   ##
=========================================
  Coverage     83.68%   83.68%           
  Complexity      494      494           
=========================================
  Files            45       45           
  Lines          1778     1778           
=========================================
  Hits           1488     1488           
  Misses          290      290

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