Update node version in gzweb_install tutorial

[jazibdawre]

Hey, the node version specified in the gzweb_install tutorial didn't work osrf/gzweb#217 osrf/gzweb#173 . I tested it on Ubuntu 18.04 amd64 with gazebo 9 and have updated the version.

v6.17.1 (npm v3.10.10): Build Error
v7.10.1 (npm v4.2.0): Build Error

gzweb/node_modules/micromatch/index.js:44
    let isMatch = picomatch(String(patterns[i]), { ...options, onResult }, true);
                                                   ^^^

SyntaxError: Unexpected token ...

v8.17.0 (npm v6.13.4): Build OK, Run OK
v9.11.2 (npm v5.6.0): Build OK, Run OK
v10.24.1 (npm v6.14.12): Build OK, Run OK [Deprecation Warnings]
v11.15.0 (npm v6.7.0): Build OK, Run OK [Deprecation Warnings]

v12.22.1 (npm v6.14.12): Build Error

In file included from ../GZNode.cc:19:0:
../GZNode.hh:36:30: error: ‘v8::Handle’ has not been declared
     public: static void Init(v8::Handle<v8::Object> exports);

The version 1.4.1 in the tutorial needs to have osrf/gzweb#205 applied so I kept it out of the list.

[iche033]

looks good to me, thanks for the updating the tutorial instructions