babel plugin adds annotations even when no @flow pragma is present

This is a:

- [x] Bug Report

Which concerns:

- [x] babel-plugin-flow-runtime

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### What is the current behaviour?

Currently, a file that contains type annotations, but no `@flow` pragma, will still get the `flow-runtime` annotations. This is a problem, especially since you cannot write `@flow-runtime ignore`, without enabling flow itself at the same time 🤓

(Flow looks for `@flow`, not `^@flow$`, and `@flow-runtime` satisfies the regex).

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### What is the expected behaviour?

The babel plugin should not annotate files without the `@flow` pragma.

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### Which package versions are you using?

0.10.0