Allow expectations on element count in Finders#all().

- all() will now wait for the number of elements returned to match
  the expected :count, :minimum, :maximum, :between query options.
- Helpers#matches_count? returns true if none of the count  query
  options are specified.
- Helpers#matches_count? tests multiple conditions if more than one
  is specified. If :count is specified, however, it ignores others.
- Matchers#assert_selector allows checking for 0 matches when
  the query options include {:count => 0}
Fix spec failures
Add specs for new query options on Finders#all method
Un-DRY specs
Use let instead of tap
Remove options hash
Use options hash from query object
Build on latest rbx version
Merge pull request #1240 from abotalov/use_rbx_on_travis

Use latest Chromedriver and rbx for Travis tests
Merge pull request #1232 from centro/fix-count-query-options

Enable use of count query options with Capybara::Node::Finders#all