Add custom path to the command

Another implementation of #131 

You can add paths to PATH environment variable in spec_helper.rb like this:

```ruby
RSpec.configure do |c|
  c.path = '/usr/local/rbenv/shims:/sbin:/usr/sbin'
  ...
end
```

Or in each spec like this:

```ruby
describe 'Custom path setting' do
  before :all do
    RSpec.configure do |c|
      c.path = '/usr/local/rbenv/shims'
    end
  end
  ...
end
```