Fix errors on macOS for WSL detection

[hexcowboy]

Relating to issues #33 and #35

The problem is solely that when detecting the OS, the file /proc/version is read. Darwin does not contain this file so it throws an error. It's better to nest the check for WSL in the s:os == "Linux" line.

The issue may still persist (#34) that an error will be thrown on WSL that don't contain the proc/version file. I don't have WSL to test.

This PR fixes the issue on macOS.

[hexcowboy]

Also if anyone stumbles here because they're trying to fix this issue on WSL, try using grep to check for WSL. And modify the IsWSL function as necessary.

The vimscript would look like:

if execute "grep -q Microsoft /proc/version"
  return 1
endif

[ferrine]

I believe we need to fix both at once, so let's wait for folks with WSL

[keblek]

I am on Neovim with macOS Cataline using this plugin and I get a flashing error where the plugin is looking for proc/version but then it goes away but the plugin seems to work.

Just to let you know.

[hexcowboy]

I was able to test on WSL and the error is no longer occuring.

However, I did find a new bug that is unrelated but pertains to WSL. I will reference it in a new Issue.

[ferrine]

Should we merge?

[hexcowboy]

Probably not.. wait for a fix for #38. I'll look into it this week to figure out what the WSL problem is.

[hexcowboy]

Ready for merging now that #38 and #34 are resolved

[ferrine]

Merged 2 pull requests, is it working fine now?