add HystrixCommand support

fixes OpenFeigngh-189

Squash changelog version

Class-level headers from inherited class when method is defined in de…

…claring class

Relates to OpenFeign#266, adds support to have @Header on the parent interface too

Supports POST without a body parameter

```java
@RequestLine("POST")
String noPostBody();
```

see
http://johnfeng.github.io/blog/2015/06/30/okhttp-updates-post-wouldnt-be-allowed-to-have-null-body/

Switches body parameter substitution to use encoded parameters

Resolves issue where some characters (+ %..) would end up double decoded if existing in parameters.

Fixes OpenFeign#264

Merge pull request OpenFeign#267 from Netflix/dont-break-on-processAn…

…notationOnClass

Un-abstracts processAnnotationOnClass so that we don't break api

Moves off legacy travis containers

Fixes NPE when apache client rebuffers content

When log level is full, the response body is rebuffered. The Apache
client had a bug where it allowed `toInputStream` to return null. This
fixes that bug and backfills tests for the other two clients.

Fixes OpenFeign#255

Bumps changelog

Bumps dependency versions for integrations

* OkHttp/MockWebServer 2.4.0
* Gson 2.3.1
* Jackson 2.6.0
* Ribbon 2.1.0
* SLF4J 1.7.12