Address issues raised by SensioLabs Insights

These are:

 - The composer.json file should be valid.
   The property branch-alias is not defined and the definition does not
   allow additional properties.

 - Logical operators should be avoided.
   In SlotMachine\SlotMachine::get(), The and operator does not have the
   same precedence as &&. This could lead to unexpected behavior, use &&
   instead.

 - In SlotMachine\SlotMachine::__construct() and in
   SlotMachine\SlotMachine::initialize(), the machine local variable is
   declared but never used. You should remove it.