Update pg to 4.5.5

[joaosa]

While coming up with BlueHotDog/sails-migrations#76, I somehow missed sails-migrations's optionalDependencies and ended up updating this project's version of node-postgres to 4.5.5.

This should fix any issues with running this on Node 6, as mentioned on brianc/node-postgres#1000.

Additionally, It appears the tests were failing to begin with. Before the fix, I got:

76 passing (3s)
2 pending
7 failing

Now I get:

80 passing (2s)
2 pending
6 failing

As this doesn't seem to add up, I'm wondering if this final part is an actual improvement or not.

[particlebanana]

It's actually already updated and published for the stable version on npm. https://github.com/balderdashy/sails-postgresql/blob/0.11.x/package.json#L20.

I'll merge this for master though so I don't forget.