fix: Fix warnings in Interactive Elixir (1.6.0) by using parentheses …

…in mix.exs

Removes the following warnings that appear when running IEx with the merkle_tree dependency:

$ iex -S mix
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

warning: variable "description" does not exist and is being expanded to "description()", please use parentheses to remove the ambiguity or change the variable name
  /Users/Ls/code/blockchain/tendermint-elixir/blockchain_tendermint/deps/merkle_tree/mix.exs:10

warning: variable "package" does not exist and is being expanded to "package()", please use parentheses to remove the ambiguity or change the variable name
  /Users/Ls/code/blockchain/tendermint-elixir/blockchain_tendermint/deps/merkle_tree/mix.exs:11

warning: variable "deps" does not exist and is being expanded to "deps()", please use parentheses to remove the ambiguity or change the variable name
  /Users/Ls/code/blockchain/tendermint-elixir/blockchain_tendermint/deps/merkle_tree/mix.exs:12

mix.exs

@@ -17,7 +17,7 @@ defmodule MerkleTree.Mixfile do
   #
   # Type "mix help compile.app" for more information
 
-  defp description do
+  defp description() do
     """
     A hash tree or Merkle tree is a tree in which every non-leaf node is labelled
     with the hash of the labels or values (in case of leaves) of its child nodes.
@@ -26,7 +26,7 @@ defmodule MerkleTree.Mixfile do
     """
   end
 
-  defp package do
+  defp package() do
     [
      files: ["lib", "mix.exs", "README.md"],
      maintainers: ["Yos Riady"],
@@ -50,7 +50,7 @@ defmodule MerkleTree.Mixfile do
   #   {:mydep, git: "https://github.com/elixir-lang/mydep.git", tag: "0.1.0"}
   #
   # Type "mix help deps" for more examples and options
-  defp deps do
+  defp deps() do
     [{:ex_doc, "~> 0.11", only: :dev},
      {:earmark, "~> 0.1", only: :dev},
      {:dialyxir, "~> 0.3", only: :dev},