vendor: remove vendoring by dep

Firstly removing vendor/ and dep
which will be followed up by a change
switching to using modules. For now
this change enables Travis CI to build.
Could have actually just updated dependencies
using dep but that'd be an extra change and
we are already revamping this repository.

Also while here fix Stackdriver import path:
This is necessary to change to be bundled with removing
dep so that Travis CI can build as the Stackdriver
exporter was migrated to:
    "contrib.go.opencensus.io/exporter/stackdriver"

Updates #33

cmd/ocagent/exporter/stackdriver.go

@@ -17,7 +17,7 @@ package exporter
 import (
 	"log"
 
-	"go.opencensus.io/exporter/stackdriver"
+	"contrib.go.opencensus.io/exporter/stackdriver"
 	"go.opencensus.io/stats/view"
 	"go.opencensus.io/trace"
 	yaml "gopkg.in/yaml.v2"