Force CircleCI to use JDK 8

Summary:
Circle CI is failing sometimes on the `buck && ant` task with a `Require JDK 8 or higher` error.

**Test plan (required)**
Make sure tests pass on Circle CI.
Closes https://github.com/facebook/react-native/pull/10390

Differential Revision: D4021546

Pulled By: bestander

fbshipit-source-id: 355657936bf90e70a10df418b378ad07978c0e5e

circle.yml

@@ -10,6 +10,8 @@ machine:
     TERM: "dumb"
     ADB_INSTALL_TIMEOUT: 10
     GRADLE_OPTS: '-Dorg.gradle.jvmargs="-Xmx512m -XX:+HeapDumpOnOutOfMemoryError"'
+  java:
+    version: 'oraclejdk8'
 
 dependencies:
   override: