Simple Travis-CI for GitHub contributions plus fixes

This MR revamps the Travis-CI build file in order to allow a quick test on GitHub to be executed. This comprises building on one platform (SLC6) with one compiler (LLVM), the formatting test and all tests except performance.

This reduced set should allow us to spot major issues immediately, while a more thorough CI should still be carried out by manually pushing the branch to GitLab before merging. How this workflow will look like exactly we will have to see.

In addition to this, there is one important fix here: our CI didn't execute the example tests because of a cyclic dependency between tests. Instead of failing, ctest is so nice to simply report:

$ ctest -R examples --output-on-failure -j4
Test project /builds/allpix-squared/allpix-squared/build
Error: a cycle exists in the test dependency graph for the test "examples/fast_simulation".
Please fix the cycle and run ctest again.
No tests were found!!!
Job succeeded

This is fixed now (was a copy-paste mistake).