Update podspec to version 0.3.1

TKFormTextField.podspec

@@ -8,7 +8,7 @@
 
 Pod::Spec.new do |s|
   s.name             = 'TKFormTextField'
-  s.version          = '0.3.0'
+  s.version          = '0.3.1'
   s.summary          = 'A UITextField subclass with built-in error label, floating label while editing, and underline.'
   s.description      = 'For text input forms, using alerts for showing error is too intrusive, makes user lost context of which field is wrong, and require another tap to dismiss. And you can only show one at a time. I think TKFormTextField is the answer to text input form UI.'
   s.homepage         = 'https://github.com/hlung/TKFormTextField'