Add VCL#set_main!

The Fastly API allows customers to upload multiple customer VCLs. The
primary ("main") VCL must be designated as such. This causes quite a bit
of confusion, particularly if the user has experience with our UI.

This is a bandaid, but it:
* exposes VCL's main attribute (boolean)
* adds a new method (`VCL#set_main!`) for designating `main`

Example Usage:
```
vcl_name = 'new_vcl'
vcl_content = File.read("./#{vcl_name}.vcl")
version.upload_vcl(vcl_name, vcl_content)
version.vcl(vcl_name).main # => false
version.vcl(vcl_name).set_main!
version.vcl(vcl_name).main # => true
```
Use `VCL#set_main!` in README
Add `Version#upload_main_vcl`