Use latest guard when changed minor version on guard

Install old guard-spork(1.5.1) when I wrote following Gemfile caused by dependency conflict.

```
source 'https://rubygems.org'

gem 'guard-rubocop'
gem 'guard-spork'
```

guard-spork require guard(2.8.x) in gemspec.
some plugin gems of guard require latest guard(2.x) in their gemspec.(ex yujinakayama/guard-rubocop )

We can pass our specs without error when use guard(2.9.0) and guard-spork(2.0.1) that fixed dependent version in gemspec manually.