Restore JDK8 compatibility

I misconfigured my IDE and noticed that we'd stopped compiling on JDK8.

Testing Done:
./gradlew -Pq build

With

```
diff --git a/build.gradle b/build.gradle
index 8f7eed0..af3d4ba 100644
--- a/build.gradle
+++ b/build.gradle
@@ -38,8 +38,8 @@ def httpAssetsPath = 'scheduler/assets'
 def thriftVersion = '0.9.1'

 compileJava {
-  sourceCompatibility = 1.7
-  targetCompatibility = 1.7
+  sourceCompatibility = 1.8
+  targetCompatibility = 1.8
 }
```

Reviewed at https://reviews.apache.org/r/26504/