Added lacking dependency

Require Apache Commons Collections for bidirectional maps.

build.gradle

@@ -72,6 +72,10 @@ dependencies {
     // Pairs and other utilities
     compile 'org.apache.commons:commons-lang3:3.0'
 
+    // BiDirectional maps
+    compile 'org.apache.commons:commons-collections4:4.4'
+
+
     implementation 'com.google.code.gson:gson:2.8.5'
     // Declare the dependency for your favourite test framework you want to use in your tests.
     // TestNG is also supported by the Gradle Test task. Just change the