Fix warnings in mix.exs for 'variable does not exist'

[smpoulsen]

This resolves the following warnings:

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:10

[folz]

Thanks very much!