Require ElasticSearch::SearchBuilder v0.18 to fix hash-order dependent
tests.

Closes #29
BUGFIX: has_changed() was comparing refs as a JSON string, then trying to
inflate the original value from that string, when it should have used
the original data structure.
update_mapping() and delete_mapping() now accept optional args, which
get passed to ElasticSearch.pm

Closes #27
Updated Travis to use ES v0.20.2