Fix warning of mix: "please use parentheses to remove the ambiguity"

[piglovesyou]

I get this without this change:

[pig@localhost elixir_v8]$ mix deps.get
warning: variable "description" does not exist and is being expanded to "description()", please use parentheses to remove the ambiguity or change the variable name
  mix.exs:8

warning: variable "package" does not exist and is being expanded to "package()", please use parentheses to remove the ambiguity or change the variable name
  mix.exs:9

warning: variable "deps" does not exist and is being expanded to "deps()", please use parentheses to remove the ambiguity or change the variable name
  mix.exs:12

(...)