Fix rspec gem dependency

Problem
=======

With the current rspec dependency definition in guard-rspec:

    s.add_dependency 'rspec', '>= 2.14', '~> 3.0.0.beta', '< 4.0'

And a Gemfile in another project:

    gem 'rspec', '~> 2.14'
    gem 'guard-rspec', '~> 4.2.2'

Then run Bundler:

    $ 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:

    s.add_dependency 'rspec', '>= 2.14', '< 4.0'

This works in another project:

    gem 'rspec', '~> 2.14'
    gem 'guard-rspec', '~> 4.2.2'

And this also works:

    gem 'rspec', '~> 3.0.0.beta1'
    gem 'guard-rspec', '~> 4.2.2'
Improve other dependencies

'~> 2.1' is equivalent of '>= 2.1', '< 3.0'.
Merge pull request #236 from yujinakayama/fix-deps

Fix dependencies