Add execjs to dependencies

[shuheiktgw]

This PR relates to #234

When I run pre-commit, it warns me Could not load execjs: cannot load such file -- execjs. I found In js plugin, pre-commit checks if it can load execjs or not. I think it is kind of inconvenient I need to manually install execjs in order to use js plugin, so added it to dependency! 👍

[jish]

Hmm... let me think about this. I'm not sure if the execjs dependency is missing on purpose or by accident. 🤔

On the one hand we want to ship a working program out of the box, so execjs should be an explicit dependency.

But on the other hand, I believe there are some projects that want to put pre-commit in their Gemfile and also do not want execjs as part of their bundle.

@shajith do you have an opinion on this?

[shuheiktgw]

Oh, now I see the intention of this code! 👍
Hmm, what if I discard this change, and just make the warning message more developer friendly to tell the developers that they need to add execjs to their dependency manually?

[jish]

"Oh, now I see the intention of this code! 👍"
"Hmm, what if I discard this change, and just make the warning message more developer friendly to tell the developers that they need to add execjs to their dependency manually?"

👍 That would be great. A better message would be helpful no matter what path we choose going forward 🙂

[shuheiktgw]

Thank you for your comment!
I just made another PR #273!