Fix context.rb to avoid URI#hostname for Ruby 1.9.2

Hi,

Ruby 1.9.2 doesn't support URI#hostname (http://ruby-doc.org/stdlib-1.9.2/libdoc/uri/rdoc/URI/Generic.html). This fix allows us to avoid URI#hostname if it's not defined and to use URI#host instead.

Alternatively, it's good idea to remove 1.9.2 support from README.md? :)

Regards,
makisyu