Fetch current rapidjson codebase rather than last release

[matterhorn103]

Version 1.1.0 of rapidjson is now 8 years old, and building Open Babel is currently broken on recent Linux systems as a result. Using the current code incorporates the many patches since 2016 such as this PR.

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[ghutchis]

No, we're not going to grab master. For consistency and security, it's much better to grab a specific commit hash. That's immutable.

I'd rather see a new release of rapidjson, but in the meantime, I'd prefer something like 7c73dd7de7c4f14379b781418c6e947ad464c818

[matterhorn103]

Fair enough, I swapped in the hash you suggested i.e. the most recent commit hash.

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