fix(.travis.yml): (try to fix) random build failures at travis

At https://bitcointalk.org/index.php?topic=304389.0, people suggest
that the failure

    g++-4.8: internal compiler error: Killed (program cc1plus)

usually happens when the system does not have enough free memory.
A possible workaround is to disable parallel build, and compile file by
file. I added the option `-j1` to Ninja.