Use !! to coerce booleans

* Whitespace
Relax ActiveRecord version requirement to ~> 4.0

This commit allows a single gem to support ActiveRecord 4.0.x and 4.1.x.

* Add a travis AR 4.0 gemfile
* Remove minitest dependency from gemspec. it is included via
activerecord
* Add a TestHelper
* Define the base test class based on minitest
* Add MainAdapter#column_type_map
* TableDefinition#initialize depends on AR version