Fix defaultValue.

grape-swagger 0.7.2 tries to map
  'defaultValue' -> 'defaultValue'

But there is no such attribute in grape 0.6.1.
This patch makes it work by changing the above to:
  'default' -> 'defaultValue'
Merge branch 'fix-defaultvalue' of github.com:m-o-e/grape-swagger

This is from pull request #87

Conflicts:
	lib/grape-swagger.rb