Upgrade macOS build for Qt 5.12

[hombit]

It should shadow the problems of #125. Now Travis build supports only macOS 10.12+

Check this GUI features before merge:

• [x] Positive and negative BITPIX images
• [x] Zoom, rotate and flip
• [x] Level control
• [x] Color maps
• [x] Open new image in the same window and in new window