Change default source in Gemfiles, use https to access rubygems.org

The source :rubygems is deprecated because HTTP requests are insecure
Merge pull request #1058 from tyabe/change_source_in_gemfile

Change default source in Gemfiles, use https to access rubygems.org
fix mailer generation to handle nested apps

in relation to #1011
Merge pull request #1060 from tyabe/fix_mailer_generation

fix mailer generation to handle nested apps