only run 'update-pam' when needed

TelekomLabs-DCO-1.1-Signed-off-by: Matthew Haughton <matt@3flex.com.au> (github: 3flex)
Merge pull request #39 from 3flex/patch-1

only run 'update-pam' when needed
1.0.2

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
merged in 1.0.2