Upgrade to proguard 5.2.1
diff --git a/build.gradle b/build.gradle
index 76f9e22..f0d1e00 100644
--- a/build.gradle
+++ b/build.gradle
@@ -91,7 +91,7 @@
                    commons_cli: 'commons-cli:commons-cli:1.2',
                    jflex: 'de.jflex:jflex:1.4.3',
                    jflex_plugin: 'co.tomlee.gradle.plugins:gradle-jflex-plugin:0.0.1',
-                   proguard_gradle: 'net.sf.proguard:proguard-gradle:5.1',
+                   proguard_gradle: 'net.sf.proguard:proguard-gradle:5.2.1',
                    dx: 'com.google.android.tools:dx:1.7'
          ]
     }