Please follow redirect responses

```
$ curl -i https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-1.9.7-linux-x86_64.tar.bz2 
HTTP/1.1 302 FOUND
Server: nginx/1.5.10
Date: Wed, 14 May 2014 10:50:27 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 0
Connection: keep-alive
Vary: Accept-Language, Cookie
X-Served-By: app23
X-Render-Time: 0.0280449390411
Content-Language: en
ETag: "d41d8cd98f00b204e9800998ecf8427e"
Location: http://cdn.bitbucket.org/ariya/phantomjs/downloads/phantomjs-1.9.7-linux-x86_64.tar.bz2
Cache-Control: max-age=0
X-Request-Count: 195
Expires: Wed, 14 May 2014 10:50:27 GMT
Last-Modified: Wed, 14 May 2014 10:50:27 GMT
X-Frame-Options: SAMEORIGIN
X-Static-Version: cd6830af9347
X-Version: 83272f0ae8b6
Strict-Transport-Security: max-age=31536000
X-Content-Type-Options: nosniff
```
Merge pull request #3 from alphagov/follow-redirects

Please follow redirect responses
Add sha checksum functionality

As a developer
I would like to know that software I’m downloading is what I expect
So that I am confident I’m not installing any old rubbish