Add OpenJDK dependency required for "jni.h"
Fixes docker build: Solves
```
100%] Linking CXX executable ../tests/model_test
[100%] Built target model_test
/code/jni/kenlm_wrap.cc:28:17: fatal error: jni.h: No such file or directory
#include <jni.h>
^
compilation terminated.
Removing intermediate container 0c2cb0c17a12
```
diff --git a/distribution/docker/kenlm/Dockerfile b/distribution/docker/kenlm/Dockerfile
index fd82090..5342275 100644
--- a/distribution/docker/kenlm/Dockerfile
+++ b/distribution/docker/kenlm/Dockerfile
@@ -28,7 +28,8 @@
liblzma-dev \
libz-dev \
make \
- curl
+ curl \
+ openjdk-8-jdk
# set environment variables
ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64/