Add weaveworks/promrus v1.1.0.

```
$ gvt fetch -tag v1.1.0 github.com/weaveworks/promrus
2017/10/08 XX:XX:XX Fetching: github.com/weaveworks/promrus
2017/10/08 XX:XX:XX · Fetching recursive dependency: github.com/julienschmidt/httprouter
2017/10/08 XX:XX:XX · Fetching recursive dependency: github.com/go-kit/kit/log
2017/10/08 XX:XX:XX ·· Fetching recursive dependency: github.com/go-stack/stack
2017/10/08 XX:XX:XX ·· Fetching recursive dependency: github.com/go-logfmt/logfmt
2017/10/08 XX:XX:XX ··· Fetching recursive dependency: github.com/kr/logfmt
2017/10/08 XX:XX:XX · Skipping (existing): gopkg.in/yaml.v2
2017/10/08 XX:XX:XX · Fetching recursive dependency: gopkg.in/alecthomas/kingpin.v2
2017/10/08 XX:XX:XX ·· Fetching recursive dependency: github.com/alecthomas/units
2017/10/08 XX:XX:XX ·· Fetching recursive dependency: github.com/alecthomas/template
2017/10/08 XX:XX:XX · Skipping (existing): golang.org/x/net/context/ctxhttp
2017/10/08 XX:XX:XX · Fetching recursive dependency: github.com/stretchr/objx
2017/10/08 XX:XX:XX · Skipping (existing): golang.org/x/net/context
2017/10/08 XX:XX:XX · Fetching recursive dependency: github.com/pkg/errors
```

vendor/github.com/stretchr/objx/doc.go

@@ -0,0 +1,72 @@
+// objx - Go package for dealing with maps, slices, JSON and other data.
+//
+// Overview
+//
+// Objx provides the `objx.Map` type, which is a `map[string]interface{}` that exposes
+// a powerful `Get` method (among others) that allows you to easily and quickly get
+// access to data within the map, without having to worry too much about type assertions,
+// missing data, default values etc.
+//
+// Pattern
+//
+// Objx uses a preditable pattern to make access data from within `map[string]interface{}'s
+// easy.
+//
+// Call one of the `objx.` functions to create your `objx.Map` to get going:
+//
+//     m, err := objx.FromJSON(json)
+//
+// NOTE: Any methods or functions with the `Must` prefix will panic if something goes wrong,
+// the rest will be optimistic and try to figure things out without panicking.
+//
+// Use `Get` to access the value you're interested in.  You can use dot and array
+// notation too:
+//
+//     m.Get("places[0].latlng")
+//
+// Once you have saught the `Value` you're interested in, you can use the `Is*` methods
+// to determine its type.
+//
+//     if m.Get("code").IsStr() { /* ... */ }
+//
+// Or you can just assume the type, and use one of the strong type methods to
+// extract the real value:
+//
+//     m.Get("code").Int()
+//
+// If there's no value there (or if it's the wrong type) then a default value
+// will be returned, or you can be explicit about the default value.
+//
+//     Get("code").Int(-1)
+//
+// If you're dealing with a slice of data as a value, Objx provides many useful
+// methods for iterating, manipulating and selecting that data.  You can find out more
+// by exploring the index below.
+//
+// Reading data
+//
+// A simple example of how to use Objx:
+//
+//     // use MustFromJSON to make an objx.Map from some JSON
+//     m := objx.MustFromJSON(`{"name": "Mat", "age": 30}`)
+//
+//     // get the details
+//     name := m.Get("name").Str()
+//     age := m.Get("age").Int()
+//
+//     // get their nickname (or use their name if they
+//     // don't have one)
+//     nickname := m.Get("nickname").Str(name)
+//
+// Ranging
+//
+// Since `objx.Map` is a `map[string]interface{}` you can treat it as such.  For
+// example, to `range` the data, do what you would expect:
+//
+//     m := objx.MustFromJSON(json)
+//     for key, value := range m {
+//
+//       /* ... do your magic ... */
+//
+//     }
+package objx