vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/github.com/dropbox/dropbox-sdk-go-unofficial/.travis.yml

@@ -6,18 +6,11 @@ go:
   - 1.8.x
   - 1.9.x
 
-install: go get -u golang.org/x/oauth2
+install:
+  - go get -u golang.org/x/oauth2
+
+before_script:
+  - go get -u github.com/mitchellh/gox
 
 script:
-  - set -e
-  - GOOS=linux   GOARCH=amd64 go install ./dropbox/...
-  - GOOS=darwin  GOARCH=amd64 go install ./dropbox/...
-  - GOOS=windows GOARCH=amd64 go install ./dropbox/...
-  - set +e
-
-jobs:
-  include:
-    - stage: run tests
-      go: 1.9.x
-      install: go get -u golang.org/x/oauth2
-      script: go test ./dropbox/...
+  - gox -osarch="darwin/amd64 linux/amd64 windows/amd64" ./dropbox/...