Travis CI: Set unit test to "NO FORK" mode

Another attempt to analyze this MacOS problem, that seems to exist only on Travis:
https://github.com/civetweb/civetweb/issues/1048

Do not fork to allow analyzing core dumps:
https://libcheck.github.io/check/doc/check_html/check_4.html#No-Fork-Mode

.travis.yml

@@ -139,7 +139,7 @@ script:
       make -f Makefile.osx package;
     fi
   - if [ "${RUN_UNITTEST}" == "1" ]; then
-      CTEST_OUTPUT_ON_FAILURE=1 make all test;
+      CTEST_OUTPUT_ON_FAILURE=1  CK_FORK=no  make all test;
     fi
   - pwd
   - ls -la unittest