Avoid bootstrap when --db-spec is sent.

This is how db-url option already works.
Chase D8 Drupal::service syntax.
Update docs to reflect var_export outputformat name.
Change 8.x-6.x to 6.x, and 7.x-5.x to 5.x in README.md
Missed 4.x and 3.x in the last commit.
Change branches to test to master, 6.x and 5.x now that 8.x-6.x and 7.x-5.x have been renamed.
Update src urls to travis build result images.
Don't call drush_set_error in API function _drush_sql_query(); call it in command callback, drush_sql_query().
Fix #44: Make default format for config-get yaml, not labeled-export.
Add column for PHP version in README