Ref: https://segment.atlassian.net/browse/BUGS-644
Ref: https://segment.atlassian.net/browse/BUGS-644

Previously the build was sent and stored as an integer. This can be
incorrect since CFBundleVersion can be a string, e.g. `4.5.2`.

The change will check if there is a value stored under the old key. If
there is, it will copied over as a string to the new key, and the old
key will be deleted.

This may send a truncated `previous_build` (i.e. "4" instead of "4.5.2")
for
the first "Application Updated" call for customers migrating from an
affected version.

Tested locally. An event with the old SDK shows build as `1`:
https://cloudup.com/cJY0mSyiLpj

An event with the new SDK shows build as `1.0.00` and previous_build as
`1` https://cloudup.com/cq5fLFpvBVr.

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