missed path quotation  in `install`

In `install`, line 25 is unquoted `built_path`:

```
 R(paste("CMD INSTALL ", built_path, " --library=", shQuote(.libPaths()[1]), 
```

could be fixed to

```
 R(paste("CMD INSTALL ", shQuote(built_path), " --library=", shQuote(.libPaths()[1]), 
```
