handle log directory not being present for DualLogger

not all projects that use pd-cap-recipes (or capistrano) have a log directory
where cap command is executed from.

this is to fix DualLogger initialization by rescuing error that may be raised
due to log directory not being present and assigning nil to file. Logger will
handle output file being passed as nil graciously.
bump version to 0.0.6
Merge pull request #22 from PagerDuty/check-logger-before-logging

handle File.open raising an error and failing