Update user-agent string to pnpm.js

lib/got.js

@@ -31,7 +31,7 @@ function extend (options) {
   if (!options) options = {}
   return assign({}, options, {
     headers: assign({}, options.headers || {}, {
-      'user-agent': 'https://github.com/rstacruz'
+      'user-agent': 'https://github.com/rstacruz/pnpm.js'
     })
   })
 }