Bump gson from 2.9.0 to 2.9.1 (#407)

Author: dependabot[bot]

build.gradle

@@ -76,7 +76,7 @@ dependencies {
     // Serialize constraints
     implementation 'com.googlecode.json-simple:json-simple:1.1.1'
     // Pretty print serialized constraints
-    implementation 'com.google.code.gson:gson:2.9.0'
+    implementation 'com.google.code.gson:gson:2.9.1'
 
     implementation 'org.ow2.sat4j:org.ow2.sat4j.core:2.3.6'
     implementation 'org.ow2.sat4j:org.ow2.sat4j.maxsat:2.3.6'