Add 'trema port_down' command. See features/trema_commands/port_down.feature for its usage and details (refs #253)
Version bump to 0.3.7
Merge branch 'release0.3.7' into develop
Make gem install work under non-default ruby path

Fixes #254

Before this commit build.rb expects 'ruby' as the interpreter path.
It leads to gem installation failure in environments where ruby 1.8
is provided as a different path such as 'ruby1.8'.
Version bump to 0.3.8
Merge branch 'release0.3.8' into develop