Fix Bundler Issue

Using Ruby 2.0, Bundler, Heroku Toolbelt and ZSH shell, I was encountering this following error:

```
$ rake deploy
.../gems/bundler-1.3.5/lib/bundler/definition.rb:361:in `validate_ruby!': Your Ruby version is 1.9.3, but your Gemfile specified 2.0.0 (Bundler::RubyVersionMismatch)
	from .../gems/bundler-1.3.5/lib/bundler.rb:116:in `setup'
	from .../gems/bundler-1.3.5/lib/bundler/setup.rb:17:in `<top (required)>'
	from /usr/local/heroku/ruby/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/local/heroku/ruby/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
Please install the heroku toolbelt or command line client
```

It seems to be documented here: https://github.com/bundler/bundler/issues/2489

This PR fixes the error by wrapping the heroku calls if Bundler is present.

There are 3 distinct commits are you may not want to accept the "clarify and standardize output" as it changes some of the formatting (for the better, I think).