In 'literal' env, don't validate that fragments exist or are used

[jtschulz]

Literal .graphql file imports are not properly reconciled, and the KnownFragmentNames and NoUnusedFragment rules will fail. This removes them from the default rules for the literal env. This is a band-aid toward #69

Note: Did not see any tests for the default rules used by envs (tests for literal generally). Happy to provide test coverage, but please advise on what kind of test you'd like to see for this :)

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests pass
• Update CHANGELOG.md with your change
• If this was a change that affects the external API, update the README

[apollo-cla]

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[stubailo]

I think this is a good improvement, since probably very few people will actually have all of the fragments in the same file. People who want to configure otherwise can always add the rule explicitly!