Update vendoring to dep from gdm (influxdata#4314)

.circleci/config.yml

@@ -15,7 +15,14 @@ jobs:
     <<: [ *defaults, *go-1_10 ]
     steps:
       - checkout
+      - restore_cache:
+          key: vendor-{{ .Branch }}-{{ checksum "Gopkg.lock" }}
       - run: 'make deps'
+      - save_cache:
+          name: 'vendored deps'
+          key: vendor-{{ .Branch }}-{{ checksum "Gopkg.lock" }}
+          paths:
+            - './vendor'
       - persist_to_workspace:
           root: '/go/src'
           paths:

.gitignore

@@ -2,3 +2,4 @@
 /telegraf
 /telegraf.exe
 /telegraf.gz
+/vendor

CONTRIBUTING.md

@@ -30,9 +30,9 @@ which can be found [on our website](http://influxdb.com/community/cla.html)
 
 Assuming you can already build the project, run these in the telegraf directory:
 
-1. `go get github.com/sparrc/gdm`
-1. `gdm restore`
-1. `GOOS=linux gdm save`
+1. `go get -u github.com/golang/dep/cmd/dep`
+2. `dep ensure`
+3. `dep ensure -add github.com/[dependency]/[new-package]`
 
 ## Input Plugins