Add support for rails 5.2, rails 6.0 and ActiveAdmin 2

• fix issue #13: xls_builder retains data between requests when there is an exception on a computed field

Drop support for ruby 1.9, rails 3.2, and ActiveAdmin 0.6.6.

Add support for rails 5.1 #8

• Add only_columns #7
• Fix typo in README.md #11
• Update tests for ActiveAdmin 1.2

This release fixes issue #1 by delaying column access until it is needed.