Move Enzyme to devDependencies
Move Enzyme to devDependencies

Summary:
**what is the change?:**
see title

**why make this change?:**
This was mistakenly added to regular dependencies, when we only use it
for testing.

**test plan:**
`rm -rf node_modules && yarn install && yarn build && yarn test`

**issue:**
None open yet, but thanks for JLarky whose comment on
https://github.com/facebook/draft-js/pull/1385#issuecomment-332989959
prompted this.
Closes https://github.com/facebook/draft-js/pull/1415

Differential Revision: D5961436

fbshipit-source-id: bb9c22bf9f29a7cfaea70d2b869b2fb17f31db8c

facebook/draft-js