Update *express* dependency

[pdehaan]

The specified version of express@~4.1.2 has a couple of known issues w/ the qs and send modules, see:
https://nodesecurity.io/advisories/qs_dos_memory_exhaustion
https://nodesecurity.io/advisories/qs_dos_extended_event_loop_blocking
https://nodesecurity.io/advisories/send-directory-traversal

Or here's the full list of outdated modules:

$ git clone https://github.com/saschagehlich/node-seo.git .

$ npm install

$ npm shrinkwrap --dev
wrote npm-shrinkwrap.json

$ # sudo npm i nsp -g
$ nsp audit-shrinkwrap
Name  Installed   Patched  Vulnerable Dependency
send    0.3.0    >= 0.8.4  seo > express
qs      0.6.6     >= 1.x   seo > express
qs      0.6.6     >= 1.x   seo > supertest > superagent

$ npm outdated --depth 0
Package     Current  Wanted  Latest  Location
express       4.1.2   4.1.2   4.9.0  express
mocha        1.18.2  1.18.2  1.21.4  mocha
should        3.3.2   3.3.2   4.0.4  should
supertest    0.12.1  0.12.1  0.13.0  supertest
underscore    1.6.0   1.6.0   1.7.0  underscore

$ travis-lint # http://lint.travis-ci.org/saschagehlich/node-seo

$ # sudo npm i pjv -g
$ pjv -wr
{ valid: true,
  warnings: [ 'Missing recommended field: contributors' ],
  recommendations: [ 'Missing optional field: engines' ] }