Calculate download_url inside the recipe

The attributes are being pre-calculated so they don't pickup
the custom version set by the user which can lead to issues
with the wrong version being installed.

If a user needs to create a custom download_url, they can
set the host and repository attributes in their wrapper cookbook
or on the chef server. For example to create the url:
```
http://example/downloads/elasticsearch-0.90.10.tar.gz
```
you set the attributes as:

```
default.elasticsearch[:version] = '0.90.10'
default.elasticsearch[:host] = 'http://example'
default.elasticsearch[:repository] = 'downloads'
```
Fixes #178