Use bin/spring in Guardfile

[JunichiIto]

I had to change Guardfile from:

guard :rspec, cmd: 'spring rspec --color --format documentation',

to:

guard :rspec, cmd: 'bin/spring rspec --color --format documentation',

Without this change, I got this error (seems Spring version unmatched):

bundle exec guard                     
09:04:46 - INFO - Guard is using TerminalTitle to send notifications.
09:04:46 - INFO - Guard::RSpec is running
09:04:46 - INFO - Guard is now watching at '/Users/jit/dev/private/rails-4-1-rspec-3-0'
[1] guard(main)> 
09:04:47 - INFO - Run all
09:04:47 - INFO - Running all specs
/Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0.rc2/lib/bundler/runtime.rb:34:in `block in setup': You have already activated spring 1.2.0, but your Gemfile requires spring 1.1.3. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0.rc2/lib/bundler/runtime.rb:19:in `setup'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0.rc2/lib/bundler.rb:120:in `setup'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/bundler-1.6.0.rc2/lib/bundler/setup.rb:7:in `<top (required)>'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/commands.rb:33:in `<module:Spring>'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/commands.rb:4:in `<top (required)>'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/server.rb:9:in `<top (required)>'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client/run.rb:42:in `block in boot_server'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client/run.rb:41:in `fork'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client/run.rb:41:in `boot_server'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client/run.rb:24:in `call'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client/command.rb:7:in `call'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/lib/spring/client.rb:26:in `run'
    from /Users/jit/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/spring-1.2.0/bin/spring:48:in `<top (required)>'
    from /Users/jit/.rbenv/versions/2.1.1/bin/spring:23:in `load'
    from /Users/jit/.rbenv/versions/2.1.1/bin/spring:23:in `<main>'