Revert Qt requirements to version 5.7 on Linux in the README.md file

[scribam]

• Partially revert dbf1573 to bring back Qt 5.7 as the minimum requirement for Linux in the README.md file
• Update the PPA urls displayed by CMake when Qt is missing

[hcorion]

Why minimum requirement for 5.8? Unless we now require features that are in 5.8, but not in 5.7, I see no reason to raise it.

[Megamouse]

someone had problem building it with 5.7

[Megamouse]

please leave the linux at 5.7 and adjust the readme for linux back to 5.7
the windows 5.8 should be correct and was the reason i changed it in the first place.

[AniLeo]

Don't spam Git, this is not a game