Use only the value of the UID in the NCX

The `@uid` variable contains an Hash with `:value`, `:scheme` and `:id` keys.
We want only the value of the identifier, not the other attributes.
Simplify ID generation
Make sure that generated IDs are XML-compatible
explicitly sets the version of rubyzip

Rubyzip v 1.0.0 has breaking changes - in that it has changed several file and class names - thus breaking the functionality of all gems that require it "the old way". 

By specifying the version explicitly this is circumvented (without the need to change the gems source code).
Merge commit 'e2ce594dae13639cb7c25acfacff3b6d395e4a77'
NCX uid issue
Merge commit 'b40d143d45504c9baffc4928ab6642a86f84648e'
Fix rubyzip version