pull docker deps beforehand

Signed-off-by: Arnav Gupta <arnav@codingblocks.com>

Author: championswimmer

.travis.yml

@@ -12,6 +12,10 @@ install:
   - npm install -D
   - docker pull codingblocks/judge-worker-c
   - docker pull codingblocks/judge-worker-cpp
+  - docker pull codingblocks/judge-worker-java8
+  - docker pull codingblocks/judge-worker-py2
+  - docker pull codingblocks/judge-worker-nodejs6
+  - docker pull codingblocks/judge-worker-nodejs8
 
 script:
   - npm run cover