Gemfile ruby version specified - added to shelly check and generated Cloudfile

```
$ shelly_local check
Checking Shelly Cloud requirements

  ✓ Gemfile is present
  ✓ Gemfile.lock is present
  ✓ jruby 1.7.4 (1.9 mode) is supported
```

```
$ shelly_local check
Checking Shelly Cloud requirements

  ✓ Gemfile is present
  ✓ Gemfile.lock is present
  ✗ Only jruby 1.7.4 (1.9 mode) is currently supported
    See more at https://shellycloud.com/documentation/requirements#ruby_versions
```

```
$ shelly_local check
Checking Shelly Cloud requirements

  ✓ Gemfile is present
  ✓ Gemfile.lock is present
  ✗ Your ruby engine: rbx is currently unsupported
    See more at https://shellycloud.com/documentation/requirements#ruby_versions
```
```
$ shelly_local check
Checking Shelly Cloud requirements

  ✓ Gemfile is present
  ✓ Gemfile.lock is present
  ✗ ruby 2.1.0 is currently unsupported
    See more at https://shellycloud.com/documentation/requirements#ruby_versions
```