Resolve build warnings

[theodowling]

Resolving warnings about unsafe variable when compiling

==> battleship
Compiling 20 files (.ex)
warning: the variable "game_data" is unsafe as it has been set inside a case/cond/receive/if/&&/||. Please explicitly return the variable value instead. For example:

    case int do
      1 -> atom = :one
      2 -> atom = :two
    end

should be written as

    atom =
      case int do
        1 -> :one
        2 -> :two
      end

Unsafe variable found at:
  lib/battleship/game.ex:95