Version bump Travis CI OS X version

[Martii]

• Current release is OS X 10.12.6 (Sierra) with Xcode 9.0 (Sep 21, 2017)
• Successful make, bundle, and open with 4c3ef116045e1e8f63427330337dc05b3bea9e12 and Qt 5.9.2 release Unified Installer installation.

Ref:

• https://docs.travis-ci.com/user/reference/osx/

Note(s):

• Xcode 9.1 not available in the App Store yet... so not sure how Travis CI is getting this.

[Martii]

Hmmm...

Re:

• https://github.com/QupZilla/qupzilla/blob/4c3ef116045e1e8f63427330337dc05b3bea9e12/.travis.yml#L32
• https://travis-ci.org/QupZilla/qupzilla/jobs/284353178#L478

$ sudo apt-add-repository -y ppa:beineri/opt-qt59-trusty
Cannot add PPA: 'ppa:~beineri/ubuntu/opt-qt59-trusty'.
The user named '~beineri' has no PPA named 'ubuntu/opt-qt59-trusty'
Please choose from the following available PPAs:
 * 'bark':  basysKom Ark
 * 'opt-qt487':  Qt 4.8.7 for /opt Precise
 * 'opt-qt487-trusty':  Qt 4.8.7 for /opt Trusty
 * 'opt-qt502':  Qt 5.0.2 for /opt Precise
 * 'opt-qt511':  Qt 5.1.1 for /opt Precise
 * 'opt-qt511-trusty':  Qt 5.1.1 for /opt Trusty
 * 'opt-qt521':  Qt 5.2.1 for /opt Precise
 * 'opt-qt521-trusty':  Qt 5.2.1 for /opt Trusty
 * 'opt-qt532':  Qt 5.3.2 for /opt Precise
 * 'opt-qt532-trusty':  Qt 5.3.2 for /opt Trusty
 * 'opt-qt542':  Qt 5.4.2 for /opt Precise
 * 'opt-qt542-trusty':  Qt 5.4.2 for /opt Trusty
 * 'opt-qt551':  Qt 5.5.1 for /opt Precise
 * 'opt-qt551-trusty':  Qt 5.5.1 for /opt Trusty
 * 'opt-qt562':  Qt 5.6.2 for /opt Precise
 * 'opt-qt562-trusty':  Qt 5.6.2 for /opt Trusty
 * 'opt-qt562-xenial':  Qt 5.6.2 for /opt Xenial
 * 'opt-qt563':  Qt 5.6.3 for /opt Precise
 * 'opt-qt563-trusty':  Qt 5.6.3 for /opt Trusty
 * 'opt-qt563-xenial':  Qt 5.6.3 for /opt Xenial
 * 'opt-qt571-trusty':  Qt 5.7.1 for /opt Trusty
 * 'opt-qt571-xenial':  Qt 5.7.1 for /opt Xenial
 * 'opt-qt58-trusty':  Qt 5.8 for /opt Trusty
 * 'opt-qt58-xenial':  Qt 5.8 for /opt Xenial
 * 'opt-qt59-xenial':  Qt 5.9.0 for /opt Xenial (will be removed soon)