when editing YAML, quote potential number literals

Summary:
This came up implementing reup for svn. Revisions are just numbers, so when we
edit them into YAML files, and then read them back in during a sync, they
become Python ints, and peru throws up. Have the editing code guard against
this and insert quotation marks as necessary. Plugins are still responsible for
only ever returning strings in their reup output.

Test Plan:
New unit test, and a new assert to guarantee plugins always return
strings when they reup.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D26