Unfortunately we can't use docker anymore in order to support automatic
build on OSX.

In order to pro-actively diagnose an issue on someone's computer, a
`omf doctor' command is being added. This command will initially check
errors that could cause a theme to not be loaded, but in the future more
checks are going to be added to prevent issues from being created for
problems that are fixable by the Oh My Fish user alone.

Instead of overwriting functions that use `exec fish` on test
environment, skip this call if the variable `CI` has been defined on the
running shell.

* Run with Travis CI, if commits contain changes to `db/themes`
* When run; fail build if `docs/Themes.md` is no longer up-to-date

Moving and renaming install_fish.sh to travis_install_fish

Moving travis_* from tests to tools.

rename: fish_user_function/ -> functions_override/

renaming travis_test.fish to test_runner.fish

Cache from 7.51MB to 1.63MB

Adding apt dependencies + build script + cache directive.