Ensure the default requirements file is used when requires is None

Ensuring that if the default requirements file is available it
gets used. Also removed an unnecessary setting of requirements
variable.

Author: jarosser06

lambda_uploader/package.py

@@ -89,13 +89,15 @@ def requirements(self, requires):
             if isinstance(requires, basestring) and \
                os.path.isfile(os.path.abspath(requires)):
                 self._requirements_file = os.path.abspath(requires)
-                self._requirements = None
             else:
                 if isinstance(self._requirements, basestring):
                     requires = requires.split()
                 self._requirements_file = None
                 self._requirements = requires
         else:
+            # If the default requirements file is found use that
+            if os.path.isfile(self._requirements_file):
+                return
             self._requirements, self._requirements_file = None, None
 
     def virtualenv(self, virtualenv):