convert time to UTC to comply with proper specific time zone

Dear u-ichi san

I might be wrong but anyways, I found a little thingie about the time of the mails that are sent using fluent-plugin-mail. The "hour" in which those mails were sent was simply not correct. I thought that my server (CentOS) was wrong but maybe the time is not properly changed to UTC. Per  ruby-doc.org/core-2.0/Time.html#method-i-strftime Time::now merely returns a time object initialized to the current system time, but we may need to give it UTC shape for emails, so I added the change. It worked well on CentOS. Regards, 

Adrian