Added rake task to fetch spec version from github and update it

I added a rake task that:
- displays the currently deployed version number of the pod
- suggests a new version number
- let the user specify a different version number
- update the podspec file

I chose to make the rake command interactive so that it can display the current released version and suggest a new version.

here is an example output:
```
$ rake release
* Running version
rake version
-- fetching version number from github
git fetch
The current released version of your pod is 0.1.1
Enter the version you want to release (0.1.2) 0.2.0
You are about to release `0.2.0`, is that correct? [y/n]
y
* Running specs
rake spec
* Linting the podspec
pod lib lint
...
```