Set -DNDEBUG for travis debug builds.

[mstemm]

Within the sysdig code there are several ASSERTS() that can occur for
error paths that aren't truly critical, such as:

17:33:52 DEBUG| [stderr] falco: /home/travis/build/draios/sysdig/userspace/libsinsp/parsers.cpp:1657: static void sinsp_parser::parse_openat_dir(sinsp_evt*, char*, int64_t, std::string*): Assertion `false' failed.

Looking at the code, it's not a truly fatal error, just an inability to
find fd information:

---

 if(evt->m_fdinfo == NULL)
 {
         ASSERT(false);
         *sdir = "<UNKNOWN>";
 }

---

When running regression tests in travis, we don't want these ASSERTs to
cause falco to exit.

To allow this, in CMakeLists.txt only set DRAIOS_DEBUG_FLAGS if it
wasn't already set, and in travis's cmake, add -DNDEBUG to
DRAIOS_DEBUG_FLAGS.