Bump to 3.8.1

Author: jyemin

build.gradle

@@ -53,7 +53,7 @@ configure(subprojects.findAll { it.name != 'util' }) {
     evaluationDependsOn(':util')
 
     group = 'org.mongodb'
-    version = '3.8.1-SNAPSHOT'
+    version = '3.8.1'
     sourceCompatibility = JavaVersion.VERSION_1_6
     targetCompatibility = JavaVersion.VERSION_1_6