Cleaned up suggester query and added missing parameters

Fixes bugs from PR 484; fix Http adapter for extraction requests

Make suggester query use suggest prefix

Fix: Remove unused imports

Fix: Cache dependencies installed with composer between builds

Fix: Remove useless override

Fix: Use more appropriate assertion

Fix: Remove useless return statements

Fix: Move assignment of default values before condition

Fix: Inline variable

Enhancement: Configure phpunit to use color

Fix: Case mismatch

Fix: Remove unused parameters

Fix: Use trusty dist

Have the suggester query use the suggest prefix as documented in the
Solr documentation. Was using spellcheck but that should be used with
a spellcheck query.

chore(travis): run travis on PHP 7.1