Add testing of oh-my-fish to the Travis CI process

[matchai]

Feature Request

Is your feature request related to a problem? Please describe.
At the moment, our CI process only assures that spacefish works as intended on one of the two most popular fish package managers. This issue aims to solve that missing coverage.

Describe the solution you'd like
Our Travis configuration would spawn two macOS CI runs and two Linux CI runs; one for fisher and one for oh-my-fish. Though this would result in longer CI runs, our CI process doesn't take more than a couple of minutes at the moment.

Describe alternatives you've considered
An alternative to installing fisher and omf on Travis directly would be to dockerize our CI runs and have Travis run the tests in those containers:
https://github.com/oh-my-fish/dockerfiles

I have attempted to address this issue in the past (#88), but have had issues installing omf every time.

[Snuggle]

After way too much testing...
https://github.com/oh-my-fish/oh-my-fish/blob/355cc597fa73c71a1638932462a30f162b69d706/bin/install#L564

This is the line that causes the buffer overflow, it's just a check to make sure the environment has all the necessary utilities for omf to function. (Coreutils mostly.)

We might be able to just comment out this line within the testing script.

[matchai]

I think we may have some luck working off of the oh-my-fish travis file: https://github.com/oh-my-fish/oh-my-fish/blob/master/.travis.yml

The issues may not occur if they use their fish Dockerfile. Definitely worth giving a shot before we start modifying their install script. 👍