have_selector replacement when capybara not installed

Fix for: https://github.com/apotonick/cells/issues/199
Added `require 'cell/base'` to spec_helper, otherwise `rspec` is falling with:
```
/home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/cells-3.10.0/lib/cell/test_case.rb:153:in `<class:TestCase>': undefined method `rails4_0?' for Cell:Module (NoMethodError)
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/cells-3.10.0/lib/cell/test_case.rb:40:in `<module:Cell>'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/cells-3.10.0/lib/cell/test_case.rb:1:in `<top (required)>'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `block in require'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:236:in `load_dependency'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:251:in `require'
	from /home/lokson/rspec-cells/spec/spec_helper.rb:8:in `<top (required)>'
	from /home/lokson/rspec-cells/spec/cells/cell_generator_spec.rb:1:in `require'
	from /home/lokson/rspec-cells/spec/cells/cell_generator_spec.rb:1:in `<top (required)>'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `each'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load_spec_files'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/command_line.rb:22:in `run'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:80:in `run'
	from /home/lokson/.rvm/gems/ruby-2.1.1@rspec-cells/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:17:in `block in autorun'
```