Requirements definition should be optional

[pdiazvargas]

Actual

If the requirements field is not defined in the manifest, when I execute juni build the process is failing with the following exeption: KeyError: 'requirements'

This is the sample definition

functions:
  sns-to-slack:
    include:
      - ./src/sns-to-slack

Expected

If the function does not define a requirements file, juni build should not fail. However, then the field is defined, the validation of the file's existence should be executed