Checker Framework: 2.8.0 -> 2.8.1 (#1879)

Author: sebright2

build.gradle

@@ -240,7 +240,7 @@ subprojects {
 
     dependencies {
         if (useCheckerFramework) {
-            ext.checkerFrameworkVersion = '2.8.0'
+            ext.checkerFrameworkVersion = '2.8.1'
 
             // 2.4.0 is the last version of the Checker Framework compiler that supports annotations
             // in comments, though it should continue to work with newer versions of the Checker Framework.