Retry npm step 5 times with 5 second pause (#18918)

dev/bots/travis_install.sh

 #!/bin/bash
 set -ex
 
+function retry {
+  local total_tries=$1
+  local remaining_tries=$total_tries
+  shift
+  while [ $remaining_tries -gt 0 ]; do
+    "$@" && break
+    remaining_tries=$(($remaining_tries - 1))
+    sleep 5
+  done
+
+  [ $remaining_tries -eq 0 ] && {
+    echo "Command still failed after $total_tries tries: $@"
+    return 1
+  }
+  return 0
+}
+
 if [ -n "$TRAVIS" ] && [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
   gem install coveralls-lcov
   gem install bundler
-  npm install -g firebase-tools@">=3.6.1 <3.7.0"
+  retry 5 npm install -g firebase-tools@">=3.6.1 <3.7.0"
 fi