Improve PHP 7.3 compatibility (#1315)

- Add PHP 7.3 to Travis configuration.
- Use more explicit typing in SpellingProcessor to avoid PHP 7.3 notice.

.travis.yml

@@ -4,6 +4,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 env:
   - VUFIND_HOME=$PWD VUFIND_LOCAL_DIR=$PWD/local

module/VuFind/src/VuFind/Search/Solr/SpellingProcessor.php

@@ -252,7 +252,7 @@ class SpellingProcessor
             foreach ($this->tokenize($query) as $token) {
                 // TODO - Do we need stricter matching here, similar to that in
                 // \VuFindSearch\Query\Query::replaceTerm()?
-                if (stripos($token, $term) !== false) {
+                if (stripos($token, (string)$term) !== false) {
                     $inToken = true;
                     // We need to replace the whole token
                     $targetTerm = $token;