fix(autocomplete|select): limit propTypes of maxHeight to string

[austingreendev]

Closes #247

Description

This PR corrects two PropType's that were incorrectly listed as allowing both string & number maxHeight values.

When we originally inlined these values with style these prop-types made sense, but with the move of these values into a styled-components context it is no longer possible, unless we use the object syntax which is available in v4.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 94.153% when pulling 7e0aecf on agreen/max-height-props into cbd748e on master.