Make `column` modify default column list

A `column` declaration now appends the declared column to the list of default columns.
Move test dependencies back to Gemfile
Fix location of Gemfile
Fix AR adapter spec
Refactor Basepack::Fields
Use AbstractAdapter when data_class is nil
Refactor columns config

Move more over from Columns to ColumnConfig. Extract common functionality from ColumnConfig and FieldConfig to AttrConfig parent class. Filter out default config options for columns to reduce JS code.
Temporarily remove travisci badge

Tests all pass locally. Need some investigation to fix 2 of them on travisci.
Merge remote-tracking branch 'upstream/master'