React error fixes (#180)

* fix className errors by replacing class= with className= in templates; remove unused imports

* fix component key warning by adding the node.id key to the component

app/src/pages/index.js

@@ -1,8 +1,6 @@
 import React from "react"
-import { Link } from "gatsby"
 
 import Layout from "../components/layout"
-import Image from "../components/image"
 import SEO from "../components/seo"
 import BookCard from "../components/bookcard"
 
@@ -10,10 +8,10 @@ export default ({data}) => {
   return (
     <Layout>
       <SEO title="Home" />
-      <div class="flex flex-wrap">
+      <div className="flex flex-wrap">
       {data.allBooksJson.edges.map(function(x) {
         return (
-          <BookCard book={x.node} />
+          <BookCard book={x.node} key={x.node.id} />
         )
       })}
       </div>