bugfix: crash when no confirmed email

Hi sishen

I found that when link to bitbucket account without any email confirmed, it's will crash with errors:

    A NoMethodError occurred in #:
     undefined method `[]' for nil:NilClass
     omniauth-bitbucket (0.0.1) lib/omniauth/strategies/bitbucket.rb:33:in `raw_info'

The root cause is that at this case Bitbucket API will not give us emails with primary as true.

the result of emails API is:


without email confirmed:

    "email"=>
     [{"active": false, "email": "bin@liubin.org",       "primary": false}]"}]

with email confirmed:

    "email"=>
     [{"active": true,  "email": "liubin0329@gmail.com", "primary": true}]"}]

 Above all ,I think my code can fix this issue and will return nil when no valid email is provided.

BR.