Fix Bitbucket pull request selection broken due to a different response

[konradstrack]

This pull request ensures correct PR selection based on Bitbucket's API, which in turn fixes submitting comments on Bitbucket PRs.

When run with the bitbucket_pr formatter: pronto run -f bitbucket_pr -r rubocop, pronto 0.7.1 fails with

/Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:68:in `block in pull': undefined method `[]' for nil:NilClass (NoMethodError)
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:68:in `each'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:68:in `find'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:68:in `pull'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:57:in `pull_id'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/bitbucket.rb:11:in `pull_comments'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/formatter/pull_request_formatter.rb:5:in `existing_comments'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/formatter/git_formatter.rb:7:in `format'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto.rb:59:in `block in run'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto.rb:58:in `each'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto.rb:58:in `run'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/cli.rb:54:in `block in run'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/cli.rb:53:in `chdir'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/lib/pronto/cli.rb:53:in `run'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
    from /Users/konradstrack/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/pronto-0.7.1/bin/pronto:6:in `<top (required)>'
    from /Users/konradstrack/.rbenv/versions/2.2.4/bin/pronto:23:in `load'
    from /Users/konradstrack/.rbenv/versions/2.2.4/bin/pronto:23:in `<main>'

The actual response from Bitbucket's API is different than the current implementation expects:

table:
  :description: ''
  :links:
    ...
  :destination:
    ...
  ...
  :source:
    commit:
      hash: 7b2e2948bf645
      links:
        self:
          href: https://api.bitbucket.org/2.0/repositories/AAA/BBB/commit/CCC
    repository:
      links:
        self:
          href: https://api.bitbucket.org/2.0/repositories/AAA/BBB
        html:
          href: https://bitbucket.org/AAA/BBB
        avatar:
          href: https://bitbucket.org/AAA/BBB/avatar/333/
      type: repository
      name: Bbb
      full_name: AAA/BBB
      uuid: "{aa2485b9-53e1-4978-882b-bf645094f734}"
    branch:
      name: feature/my-branch
  ...

even though it's not properly documented (https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Busername%7D/%7Brepo_slug%7D/pullrequests/%7Bpull_request_id%7D).

This change selects the correct source branch for comparison with @repo.branch.

[mmozuras]

@konradstrack thanks 🙇!