Remove applications key from MixProject.application/0

[Sgoettschkes]

The applications key in OpenAI.MixProject.application/0 is most likely a relict from older mix versions. I don't think it does serve a purpose anymore and we get compiler warnings in our project when compiling this library.

openai.ex/mix.exs

Lines 22 to 23 in dd48493

	applications: [:httpoison, :jason, :logger],
	extra_applications: [:logger]

If somebody can confirm that there is no specific reason to keep the applications key around, I'm happy to do a PR!