Clean up package.json from previous runs

Else wasm-pack crashes trying to read its own (old) file

src/command/utils.rs

@@ -37,6 +37,7 @@ fn find_manifest_from_cwd() -> Result<PathBuf, failure::Error> {
 
 /// Construct our `pkg` directory in the crate.
 pub fn create_pkg_dir(out_dir: &Path) -> Result<(), failure::Error> {
+    let _ = fs::remove_file(out_dir.join("package.json")); // Clean up package.json from previous runs
     fs::create_dir_all(&out_dir)?;
     fs::write(out_dir.join(".gitignore"), "*")?;
     Ok(())