Add i386: Intel 32-bit case to Travis CI

[junaruga]

Hello.

This PR is related to #359 .
It's to add Intel 32-bit (i386) case to Travis CI.

Here is my forked repository's Travis log.
There is one test failure.

https://travis-ci.org/junaruga/MACS/jobs/654689880#L1144

 checking macs2_run_callpeak_narrow/run_callpeak_narrow0.log...
...
 ... error/warning found!

The Intel 32-bit case is executed on "ubuntu" bionic i386 specific binary docker container image with QEMU on Travis x86_64 using multiarch/qemu-user-static.

You can debug on your local environment like this, if you are using Linux as your host environment.

docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
docker build --rm --build-arg BASE_IMAGE="i386/ubuntu:bionic" -t macs -f test/Dockerfile .

docker run --rm -it macs bash

[taoliu]

Nice! Thank you so much, @junaruga !

[junaruga]

Thank you for merging!