Fix defined? check on ENV

I got this Exception after upgrading to the lastest version of rvm-capistrano:

```
/.rvm/gems/ruby-2.1.2/gems/rvm-capistrano-1.5.2/lib/rvm/capistrano/base.rb:84:in `block (3 levels) in <top (required)>': undefined method `gsub' for nil:NilClass (NoMethodError)
  from /.rvm/gems/ruby-2.1.2/gems/capistrano-2.15.5/lib/capistrano/configuration/variables.rb:87:in `call'
```

The defined? check on the ENV variable always evaluates to true.
```
2.1.2 :001 > puts "defined!" if defined?(ENV['foobar'])
defined!
 => nil 
```

I guess a nil-check should be sufficient, right? That's what I did in this PR.