updatedeps target.

I'm running packer from HEAD, and every time I pull I get dependency-related errors, ie

```
==> Installing dependencies
==> Building
--> Installing dependencies to speed up builds...
# github.com/mitchellh/packer/builder/amazon/common
builder/amazon/common/block_device.go:36: unknown ec2.BlockDeviceMapping field 'NoDevice' in struct literal
make: *** [all] Error 2
```

I don't think updating deps should be default, but this will provide a lot of utility.