Fixed 'Branch develop (at ) deployed' message 

Hello,

I have found the following trouble in your gem.

If I set a custom branch name 
```
set :branch, 'develop'
```
then I get the following messages in 'revisions.log
```
Branch develop (at ) deployed as release 20141224180110 by kgovorukha
```
but I think I should get something like this:
```
Branch develop (at 12312312209afa0a5d2a6e29e08b9a5caa123123) deployed as release 20141224180110 by kgovorukha
```

Also 'current/REVISION' file is empty if I set a custom branch.
But after my change it contains a hash of commit which was deployed by Capistrano.