Updating to use git.io short url

I've added an alias to git.io the github url shortening service from git.io/get_helm.sh -> https://raw.githubusercontent.com/helm/helm/master/scripts/get

Like this:
```
$ curl -i https://git.io/ -F url=https://raw.githubusercontent.com/helm/helm/master/scripts/get -F code=get_helm.sh

HTTP/1.1 201 Created
Server: Cowboy
Connection: keep-alive
Date: Fri, 19 Apr 2019 23:20:39 GMT
Status: 201 Created
Content-Type: text/html;charset=utf-8
Location: https://git.io/get_helm.sh
Content-Length: 62
X-Xss-Protection: 1; mode=block
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Runtime: 0.008310
X-Node: f065811c-e0e8-4384-bf09-9d9d28586c4d
X-Revision: 392798d237fc1aa5cd55cada10d2945773e741a8
Strict-Transport-Security: max-age=31536000; includeSubDomains
Via: 1.1 vegur
```
I think it's a little easier to remember and use.
There is also no way to overwrite or modify alias. Once it's there it's there in perpetuity.

Thanks!

Signed-off-by: Duffie Cooley <dcooley@heptio.com>

docs/install.md

@@ -63,18 +63,18 @@ scoop install helm
 ### From Script
 
 Helm now has an installer script that will automatically grab the latest version
-of the Helm client and [install it locally](https://raw.githubusercontent.com/helm/helm/master/scripts/get).
+of the Helm client and [install it locally](https://git.io/get_helm.sh).
 
 You can fetch that script, and then execute it locally. It's well documented so
 that you can read through it and understand what it is doing before you run it.
 
 ```
-$ curl https://raw.githubusercontent.com/helm/helm/master/scripts/get > get_helm.sh
+$ curl -LO https://git.io/get_helm.sh
 $ chmod 700 get_helm.sh
 $ ./get_helm.sh
 ```
 
-Yes, you can `curl https://raw.githubusercontent.com/helm/helm/master/scripts/get | bash` that if you want to live on the edge.
+Yes, you can `curl -L https://git.io/get_helm.sh | bash` that if you want to live on the edge.
 
 ### From Canary Builds