travis: fix segfault for clang 8

The travis clang-8 reports a SIGILL with the code:

    std::string str = "    ";
    Ensures(baryonyx::left_trim(str) == "");

Fix the problem using .empty() instead of operator==.

- travis:
  - use `-O2` seems to fix a segfault in the testlib unit test.
  - execute test with gdb
1 job for test in 3 minutes and 13 seconds (queued for 1 second)