`load_from_yaml': undefined method `safe_load' for Psych:Module

Hi. I'm not really a rubist, so am not sure about the best way to investigate this. On starting a new bash session, I get:

```
/Library/Ruby/Gems/2.0.0/gems/colorls-0.1.7/lib/colorls/load_from_yaml.rb:4:in `load_from_yaml': undefined method `safe_load' for Psych:Module (NoMethodError)
from /Library/Ruby/Gems/2.0.0/gems/colorls-0.1.7/lib/colorls/flags.rb:52:in `set_color_opts'
from /Library/Ruby/Gems/2.0.0/gems/colorls-0.1.7/lib/colorls/flags.rb:5:in `initialize'
from /Library/Ruby/Gems/2.0.0/gems/colorls-0.1.7/exe/colorls:4:in `new'
from /Library/Ruby/Gems/2.0.0/gems/colorls-0.1.7/exe/colorls:4:in `'
from /usr/local/bin/colorls:23:in `load'
from /usr/local/bin/colorls:23:in `'
```
As I said, I don't write ruby regularly, so I'm sure aspects of my ruby setup are out of date -- I had to upgrade rvm, and it took a few tries with brew before I managed to get ruby 2.4.1 installed.

This only happens on init, and colorls works fine other than that.
It's a nice project, BTW. : )

Start with lib/colorls/load_from_yaml.rb and trace its call from lib/colorls/flags.rb, then reproduce the error when starting a new bash session with colorls. Done means initialization no longer raises the Psych safe_load error while colorls continues to work.

Written by the indexing model from the issue text.