`vue/prop-name-casing` warns variable computed property

[ota-meshi]

Tell us about your environment

• ESLint Version:
  4.18.1
• eslint-plugin-vue Version:
  4.3.0
• Node Version:
  8.0.0
  Please show your full configuration:

    'vue/prop-name-casing': 'error',

What did you do? Please include the actual source code causing the issue.

        export default {
          props: {
            [greeting_text]: String
          }
        }

What did you expect to happen?

Since [greeting_text] is a variable computed property, it should not be a warn.

What actually happened? Please include the actual, raw output from ESLint.

[greeting_text] warned

Prop "greeting_text" is not in camelCase.

I noticed when I was PR #402.
#402 (review)