move babel-core to devDependencies

[nescalante]

our security scans found that there is a vulnerability found on this package since this is using an old version of babel-core, which uses an old version of the json5 package (0.5.1)

├─┬ babel-plugin-jsx-display-if@3.0.0
│ └─┬ babel-core@6.26.3
│   └── json5@0.5.1

we found that babel-core is not being used as a dependency so the fix is to move it to devDependencies

[hartzis]

🙌 thanks for the PR, but don't want to get your hopes up, have no idea who has write access or publish access anymore. may be easier to fork and build for your needs 😅😸

[nescalante]

from what I saw, the one with write access is still donabrams