Update gitlab CI config

Add apt caching and fix syntax.
1 job for master in 7 minutes and 13 seconds (queued for 1 second)
Name Stage Failure
failed
build Test There is an unknown failure, please try again
$ mkdir -p build
$ cd build
$ cmake ../
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
CMake 3.1.0 or higher is required. You are running version 3.0.2


-- Configuring incomplete, errors occurred!
ERROR: Build failed: exit code 1