Configure Travis CI to build against all common Ruby versions
OrderedHash is also case insensitive

OrderedHash is a class HTTY::Cookies that is screeming to come out, in
the meantime to fix #33 this should suffice
Pull up some common headers methods in Payload

The intent is to remove duplication between HTTY::Requst and
HTTY::Response

Request do a lot of header manipulation and most of the logic should
belong to an Headers class, for now we will keep that low and continue
with small progress, a big refactoring is needed
Merged OrderedHash with Headers