Use system Qt >= 5.6

[ariya]

Qt 5.6, released on March 2016, is a good LTS version of Qt to rely on. It is already available as a stable package for major Linux distributions.

There is the most recent LTS one, Qt 5.9 (released on May 2017), but it's going to take some time until it is widely adopted by the major Linux distributions.

Thus, for the future version of PhantomJS, we should aim to mandate the official use of Qt >= 5.6 and ensure that it is tested thoroughly with Qt 5.6. This means we can support the following Linux distributions (or newer):

• CentOS 7.4: default Qt is 5.6.2
• CentOS 6.9: default Qt is 5.6.1
• Debian 9.3 (stretch): default Qt is 5.7.1
• Ubuntu 17.10 (artful): default Qt is 5.9.1

although unfortunately we need to drop the support for the older ones:

• Debian 8.10 (jessie): default Qt is still 5.3.2
• Ubuntu 16.04 LTS (xenial): default Qt is still 5.5.1

The situation with Ubuntu 16.04 is a bummer, though perhaps it is sufficient to recommend the users to get updated Qt 5.6.1 from Kubuntu PPA backports.

[ariya]

This has been implemented.