How do I update development dependencies with NPM?

Here's the part of my package.json:

{
    ...
    "devDependencies": {
        "gulp": "~3.8",
        ...
    },
    ...
}

I'm running the following commands:

gulp --version

CLI version 3.8.7
Local version 3.8.6

npm update
gulp --version

CLI version 3.8.7
Local version 3.8.6

rm -Rf ./node_modules/
npm install

gulp --version

CLI version 3.8.7
Local version 3.8.7

The npm update command has no effect.

It's only after I manually delete the node_modules directory and run npm install development packages are updated. What is the reason for this? Is it possible to actually update development packages without such a hassle?