Quick bug fix and some repo improvements

I tried to run a simple query against the Xively API and got the following error:

```
❯ irb                                                                                                                                                                                                                                                                           1.9.3-p392
irb(main):001:0> require "xively-rb"
=> true
irb(main):002:0> client = Xively::Client.new("ommitted")
=> #<Xively::Client:0x007ff68511e9e0 @api_key="omitted">
irb(main):003:0> client.get("https://api.xively.com/v2/feeds/")
NameError: uninitialized constant Xively::VERSION
	from /Users/Chris/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/xively-rb-0.2.09/lib/xively-rb/client.rb:10:in `user_agent'
	from /Users/Chris/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/xively-rb-0.2.09/lib/xively-rb/client.rb:38:in `parse_options'
	from /Users/Chris/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/xively-rb-0.2.09/lib/xively-rb/client.rb:18:in `get'
	from (irb):3
	from /Users/Chris/.rbenv/versions/1.9.3-p392/bin/irb:12:in `<main>'
irb(main):004:0>
```
I have added a `require` for the version file in `xively-rb.rb` to fix this.

I've also:

- Renamed the `.rbenv-version` file to `.ruby-version` and remove the `ruby` prefix to stop rbenv giving me warnings (this is on 0.4.0 of rbenv).
- Added some badges for the repo
- Added license to gemspec to stop `bundler build gem` from giving me warnings.