Ensure proptypes is library dependency instead of peer dependency

Author: aeagle

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "react-spaces",
-	"version": "0.1.27",
+	"version": "0.1.28",
 	"main": "dist/index.js",
 	"module": "dist/es/index.js",
 	"types": "dist/index.d.ts",
@@ -27,7 +27,8 @@
 	"license": "MIT",
 	"description": "An easy to understand and nestable layout system, React Spaces allow you to divide a page or container into anchored, scrollable and resizable spaces enabling you to build desktop type user interfaces in the browser.",
 	"dependencies": {
-		"css-element-queries": "^1.2.1"
+		"css-element-queries": "^1.2.1",
+		"prop-types": "^15.7.2"
 	},
 	"devDependencies": {
 		"@babel/core": "^7.5.5",
@@ -69,8 +70,7 @@
 	},
 	"peerDependencies": {
 		"react": "^16.8.0",
-		"react-dom": "^16.8.0",
-		"prop-types": "^15.7.2"
+		"react-dom": "^16.8.0"
 	},
 	"scripts": {
 		"start": "rollup -c -w",