Old version cmake build fails

[reoxey]

reference #40 I tried building the project in cmake 3.2.2 but gives errors like

• cannot run .c
• threads not found but when i upgraded to cmake 3.8.2, the build was successfull so right now i'm using gcc - 7.1.0 & boost - 1.61

it will better to provide the least version of cmake supported, for those who are new to this repo.

Regards.

[mrbald]

@reoxey - many thanks for root causing it, I have updated the top level CMakeLists.txt with correct minimum version spec -> https://github.com/venediktov/vanilla-rtb/commit/7141606ced4b493e64ff273e11d37f9efaa99862