ibrdtn: Do not modify time if clock rating is equal to 1.0

Clock with a rating of 1.0 are considered as reference node and should never
change their value by data received through the DTN. Instead external syncronization
approached should adjust the local clock.
Merge pull request #70 from morgenroth/Do-not-change-time-on-reference-nodes

ibrdtn: Do not modify time if clock rating is equal to 1.0