pin puppetlabs-ntp to fix notify issue

Currently, NTP service is not restarted when configuring ntp.conf.
A pull-request is in progress [1], but while this time, let's pin our
branch.

[1] https://github.com/puppetlabs/puppetlabs-ntp/pull/203

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>