Fix dependencies

## Problem

With the current `rspec` dependency definition in `guard-rspec`:

```ruby
s.add_dependency 'rspec', '>= 2.14', '~> 3.0.0.beta', '< 4.0'
```

And a `Gemfile` in another project:

```ruby
gem 'rspec', '~> 2.14'
gem 'guard-rspec', '~> 4.2.2'
```

Then run Bundler:

```bash
$ bundle update
Fetching gem metadata from https://rubygems.org/.........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "rspec":
  In Gemfile:
    rspec (~> 2.14) ruby

    guard-rspec (~> 4.2.2) ruby depends on
      rspec (3.0.0.beta1)
```

This means the `~> 3.0.0.beta` negates the `>= 2.14`.

## Solution

If `guard-rspec` defines:

```ruby
s.add_dependency 'rspec', '>= 2.14', '< 4.0'
```

Then this works in another project:

```ruby
gem 'rspec', '~> 2.14'
gem 'guard-rspec', '~> 4.2.2'
```

And this also works:

```ruby
gem 'rspec', '~> 3.0.0.beta1'
gem 'guard-rspec', '~> 4.2.2'
```

I've tested these behavior with [my gem](https://github.com/yujinakayama/test_site/blob/v0.0.2/test_site.gemspec#L21).

This fixes #235.