Move from glide to dep (cadence-workflow#1010)

Dep generates a folder, _vendor-*, with copies of the source code for dependencies. Added this to .gitignore and set the "make copyright" check to skip that folder.

Pruning is not enabled in the toml since that deletes files used during build. Updated contribution guide to talk about dep instead of glide. Set cadence version to 0.7.2

.gitignore

@@ -9,6 +9,7 @@
 .tmp/
 .vscode/
 /vendor
+/_vendor-*
 /cadence
 .DS_Store
 test

CONTRIBUTING.md

@@ -20,12 +20,11 @@ cd $GOPATH/src/github.com/uber/cadence
 
 ## Dependency management
 
-Dependencies are tracked via `glide.yaml`. If you're not familiar with `glide`,
-read the [docs](https://github.com/Masterminds/glide#usage).
-After you install gide, run below command to get all dependencies into vendor folder.
-```bash
-glide up
-```
+Dependencies are recorded in `Gopkg.toml` and managed by dep. If you're not 
+familiar with dep, read the [docs](https://golang.github.io/dep/). The Makefile 
+will call `install-dep.sh` to install dep on Mac or Linux. You can use this 
+script directly, but it will be run automatically with  `make` commands. To 
+check dependencies, run `dep ensure`. 
 
 ## Licence headers