stop roll() from failing if the new cache folder is not created (#243)

* stop roll() from failing if the new cache has no files

Today I stumbled upon an error where it would fail when the cache folder was not present
```(typescript) Error: ENOENT: no such file or directory, rename 'REDACTED\node_modules\.cache\rollup-plugin-typescript2/rpt2_759e2fd8d3f20c1a0805faf5404af6bea36632fd/code/cache_' -> 'REDACTED\node_modules\.cache\rollup-plugin-typescript2/rpt2_759e2fd8d3f20c1a0805faf5404af6bea36632fd/code/cache'```

This PR fixes it by checking its existence before trying to rename it

* use existsSync instead of fs.existsSync

* use renameSync instead of fs.renameSync

Author: xaviergonz

src/rollingcache.ts

@@ -98,6 +98,8 @@ export class RollingCache<DataType> implements ICache<DataType>
 
 		this.rolled = true;
 		removeSync(this.oldCacheRoot);
-		renameSync(this.newCacheRoot, this.oldCacheRoot);
+		if (existsSync(this.newCacheRoot)) {
+			renameSync(this.newCacheRoot, this.oldCacheRoot);
+		}
 	}
 }