Update latest version in docs.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198882730

README.md

@@ -6,13 +6,13 @@ A library for testing javac compilation with or without annotation processors. S
 Latest Release
 --------------
 
-The latest release is version `0.13`.  Include it as a [Maven](http://maven.apache.org/) dependency with the following snippet:
+The latest release is version `0.15`.  Include it as a [Maven](http://maven.apache.org/) dependency with the following snippet:
 
 ```
 <dependency>
   <groupId>com.google.testing.compile</groupId>
   <artifactId>compile-testing</artifactId>
-  <version>0.13</version>
+  <version>0.15</version>
   <scope>test</scope>
 </dependency>
 ```