Support any Moodle DB.
Main changes:
- lib/mergeusertool.php:
- revisited to clean SQLs for db-based queries
- simplified settings management
- removed warning on duplicated entries on grade items
- .travis.yml:
- updated to latest version of moodle-plugin-ci projecte (renamed)
- optimized to use specific service versions on every Travis matrix
- Testing from M3.4 up to M3.7 and master on Travis CI
- lang/en/tool_mergeusers.php:
- remove unused string