Fix bugs in Travis scripts

  * Use `make --silent` to hide not needed details from the build
  * Dropped `-fno-delete-null-pointer-checks` — if the code does not work without them, we are doing something wrong and this needs to be fixed. Fedora and other Linux distros will drop that flag anyway
  * Added `-std=gnu90` flag to catch some things Windows compiler won't be happy with
  * Fixed variable reference in unit-tests/ci/install_prereqs.sh
  * Removed Mongo hack from the code