Remove dependency on Defaults in config/optional.

[rosiel]

What does this Pull Request do?

Provide a brief description of what the intended result of the PR will be and/or what
problem it solves.

• Related GitHub Issue: Installing this on top of a blank starter or starter_dev site results in an error #11

• Other Relevant Links: (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

What's new?

Removes dependency of a specific config on islandora defaults.

How should this be tested?

You should be able to install the module and (if you have the starter site) see the mirador view mode for islandora_object? (i'm actually not sure)

Documentation Status

• Does this change existing behaviour that's currently documented? no
• Does this change require new pages or sections of documentation? no
• Who does this need to be documented for? no
• Associated documentation pull request(s): ___ or documentation issue ___

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

@noahwsmith

[noahwsmith]

#11 was solved by simply having 2.2.2 https://github.com/Islandora/islandora_mirador/releases/tag/2.2.2 available as a tag. Removing the dependency is extra gravy!