Remove update instructions

Oh My Fish has been stable for many months, we no longer need the update
instructions on the main README.

Update package: fasd

Note for fasd:

Existing fasd plugin doesn't even work properly.

- No fasd_cd
- No default aliases (f, a, s, d, z, zz, ...)

So I forked it and totally rewrote it. Please consider it as different
plugin.

New theme: shellder

Shellder is a fork of agnoster, with smarter functionalities

Fix issue #197: quote OMF_PATH

Support omf- prefix for packages

bugfix: add missing $s in variable tests in git_ahead.fish

Run tests on osx

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

Better test output

OSX `date` does not support milliseconds output, so @derekstavis created
this `C` code to generate the time with milliseconds.

Show name of the test that failed with the failure message.

Better organized 'omf theme' output

Headings are underlined.
Current theme is highlighted in the Installed category.

Test suite

registry: add oh-my-fish/plugin-balias

add powerline

rename plugin

registry: Add iex&mix plugins

Until omf support omf- prefix.

Faster CI builds

Tell Travis this is not Ruby codebase.

Add homebrew command not found plugin

omf/templates: Align README headers based on omf's