Install trema using gem package

Using trema gem package is a usual way to install a released version
of Trema.

Change-Id: I3990e087f51e67707f98627cd02db7b788e28c94
Remove unnecessary dependency packages from files/apts/trema

Change-Id: I3abd4e893667a45fdf499f2de3faeb22c2f8c6ab