Make Error Prone and FindBugs annotations 'compileOnly' dependencies.

This is part of issue #1081.
diff --git a/build.gradle b/build.gradle
index c01142f..bebb0ff 100644
--- a/build.gradle
+++ b/build.gradle
@@ -215,8 +215,8 @@
             compileOnly libraries.auto_value
         }
 
-        compile libraries.errorprone,
-                libraries.jsr305
+        compileOnly libraries.errorprone,
+                    libraries.jsr305
 
         testCompile libraries.guava_testlib,
                 libraries.junit,