ci: fix pytorch/mxnet build failure

[jasperzhong]

1. sudo apt-get install cuda chooses cuda-10.2 by default. cuda-10.1 has to be added explicitly.
2. building pytorch needs c++14.
3. python2.7 needs future to import builtins.

[jasperzhong]

4. pytorch has some updates about THCState_getCurrentStream. see remove uses of deprecated torch API (removed on commit #33376 on pyto… horovod/horovod#1731

[jasperzhong]

5. propagate exceptions from children threads to the parent thread to show failure. or CI would always pass successfully...

[bobzhuyb]

Thanks for the contribution.

@pleasantrabbit Please test this PR with our internal jobs

[jasperzhong]

byteps needs GPU to work but travis-ci does not have GPU instances. so only building and ps-lite can be tested. the lastest commit passes both. https://github.com/vycezhong/byteps/runs/622248551

[pleasantrabbit]

Thanks for the contribution.

@pleasantrabbit Please test this PR with our internal jobs

Sure. I created a test, it's been running for half an hour. Should be fine.

[bobzhuyb]

@vycezhong Thanks for the contribution.