build:installNpmDeps is now silent

Without this fix, the build fails with "stderr maxBuffer exceeded". Probably because new npm version is more verbose and outputs more information, the child process buffer is filled earlier. I started seeing the build fail after I merged elastic#6433 (I am following the 4.4 branch but I believe this applies everywhere).

A workaround is to increase the buffer size, or disable the output altogether. I chose silent mode because all output from this command is ignored anyway.

I am not sure if/why the official build works, though.

tasks/build/installNpmDeps.js

@@ -5,7 +5,7 @@ module.exports = function (grunt) {
   grunt.registerTask('_build:installNpmDeps', function () {
     grunt.file.mkdir('build/kibana/node_modules');
 
-    exec('npm install  --production --no-optional', {
+    exec('npm install  --production --no-optional --silent', {
       cwd: grunt.config.process('<%= root %>/build/kibana')
     }, this.async());
   });