expose a will_run? method

in my `spec_helper.rb` i would like to disable coveralls for regular rspec runs (better performance, no noise).

with the `will_run?` method, i can do something like this:

```ruby
require 'coveralls'
Coveralls.wear!('rails') if Coveralls.will_run?
```

if you think that there is a better solution to this, please give me some feedback.