This version contains a major breaking change to web.C, Goji's context
object: the Env key has been changed from a map[string]interface{} to a
map[interface{}]interface{} to allow for packages to use package-private
types to namespace their environment keys.

In practice, most users of Goji will be unaffected by this change:
strings that were previously in use as map keys will continue to work.
However, users (especially middleware authors) who construct Env's (with
make or similar) will have to update their code as instructed by the
compiler.