[Cython] Cython sometimes fails to build on Travis

[Ondrej Certik]

Hi,

Cython sometimes fails to build on Travis for various projects. If it
fails, it always ends with this error:

https://gist.github.com/certik/08f16dd572170c17d956

Here are some instances of this failure in other projects:

https://github.com/sympy/csympy/issues/308
https://github.com/scipy/scipy/pull/4126#issuecomment-62162812

Any ideas what is going on?

Thanks,
Ondrej