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
Headers are ordered and case insensitive

Merged `OrderedHash` with `Headers` so that headers will be case
insensitive. Thought I did that back in issue-33 branch but did not
:smile:

Also removed previous `OrderedHash` stil cited in documentation that
needs to be regenerated
Display#show_headers must be case insensitive

Headers match must be case insensitive everywhere, maybe we should put
this logic in `Headers` class?
Extract custom matcher in spec helper
Merge pull request #116 from gabrielelana/case-insensitive-headers

Case Insensitive Headers
Require RubyGems before using Gem::Version

Ruby v1.8.7 does not load RubyGems automatically.