Changed placeholder instance name for conflict with Formotion

# Problem
`placeholder` instance variable conflicts with Formotion's UITextView enhancement https://github.com/clayallsopp/formotion/blob/master/lib/formotion/patch/ui_text_view_placeholder.rb#L37. 

# Solution
This patch will change `HTBPlaceholderTextView` 's `placeholder` instance to `placeholderText`.