Make prop-types declaration file a dependency

[thchia]

Description

Moved @types/prop-types from devDependencies to dependencies.

Addressed issue #15

TypeScript consumers of the library need this package due to the type export in utils.d.ts. Currently as it is a devDependency it is not included in a normal install and results in TS compiler errors.

I have followed the official suggestion here of including it as a regular dependency.

Our package exposes declarations from each of those, so any user of our [browserify-typescript-extension - react-skeletor] package needs to have these dependencies as well. For that reason, we used "dependencies" and not "devDependencies", because otherwise our consumers would have needed to manually install those packages.

[alex3165]

@thchia Thanks for the PR, can you also move @types/react, the same should apply to this dependency then