sup-config: call to_s() on user input

Highline seems to use its own string wrapper for data entered by the
user. When converting to yaml, the value is annotated with
'!str:HighLine::String', which leads to errors when starting sup or
sup-sync.

Fixes issue #28
Merge pull request #29 from ericweikl/sup-config-bug28

sup-config: call to_s() on user input