Check whether npm is installed

Summary:
We have nice check for npm command, but we do not use it.
It will print nice warning in case npm is missing

Reviewed By: antiarchit

Differential Revision: D6536544

fbshipit-source-id: 501189e77bd927ffd646c695995346fbc2eef133

Author: Marek Cirkos

Scripts/bootstrap.sh

@@ -54,6 +54,7 @@ function fetch_and_build_dependencies() {
 
 function build_inspector() {
   echo -e "${BOLD}Building Inspector"
+  assert_has_npm
   CURRENT_DIR=$(pwd)
   RESOURCE_BUNDLE_DIR="$CURRENT_DIR/Resources/WebDriverAgent.bundle"
   INSPECTOR_DIR="$CURRENT_DIR/Inspector"