Explicity #close each zip member file that has been opened via #fopen.

Addresses a memory leak with the zipruby gem where using `fopen()` and `#read` without explicitly closing the file afterward will never release the memory used reading the file. This pull request uses `#fopen()` with a block so that zipruby will close the file after reading and free the memory.

I do not know if you would prefer this on one line instead, as:

```ruby
raw_content = Zip::Archive.open(manifest.package.book.epub_file) {|zip| zip.fopen(entry_name) {|member| member.read} }
```

I will change this if you prefer.