Rename gem from libxml-ruby to libxml

It makes sense for this repository to be named `libxml-ruby`, to avoid ambiguity with other XML libraries, but the gem name need not include `-ruby`, since the `rubygems` package manager is tightly coupled to the Ruby language. Just as the [`sqlite3-ruby` gem was renamed to `sqlite`](https://github.com/sparklemotion/sqlite3-ruby/commit/59855b543f2506829403c1d91f9c93c84298ade0), this package’s name should just be `libxml`.

This also increases consistency between the library’s name and the way it is required.

```ruby
require 'libxml'
```