Make GitLab CI work.

Switch to using openjdk image instead of sbt-firefox-chromium.
The latter defines an ENTRYPOINT, which doesn't work with GitLab CI.
gitlab-org/gitlab-ci-multi-runner#1421

Switching to openjdk image requires installing the dependencies
manually. :(

.gitlab-ci.yml

@@ -1,6 +1,15 @@
-image: gshakhn/sbt-firefox-chromium
+image: openjdk:8-alpine
+
+variables:
+  SBT_VERSION: "0.13.13"
+  SBT_PATH: "/opt/sbt-launcher-packaging-${SBT_VERSION}/bin"
+
+before_script:
+  - apk --no-cache add bash curl
+  - mkdir /opt
+  - curl -SL https://dl.bintray.com/sbt/native-packages/sbt/${SBT_VERSION}/sbt-${SBT_VERSION}.tgz | tar xz -C /opt/
 
 scalastyle:
   stage: build
   script:
-    - scalastyle
+    - $SBT_PATH/sbt scalastyle