fix if-expression for user-id check

[4ch1m]

Hi there,

If run via "/bin/sh" (instead of "/bin/bash") the first if-expression of that script will always fail with...

./install-coap-client.sh: 3: [: Illegal number: 

... because you're comparing a string with an integer.

$(id -u) should be used instead.

Thanks.

[ggravlingen]

Thanks!