File.exists? is now deprecated in Ruby, use File.exist? instead

Hello,

`File.exists` is deprecated on recent versions of Ruby.
This commit removes some warnings when we load dotenv in a project.
`warning: File.exists? is a deprecated name, use File.exist? instead`

Here is the deprecation code in the Ruby repository:
https://github.com/ruby/ruby/blob/trunk/file.c#L1462