Use official sbt distribution

Ref travis-ci/travis-ci#9816

.travis.yml

@@ -8,6 +8,14 @@ apt:
   packages:
     - openjdk8
 
+addons:
+  apt:
+    sources:
+      - sourceline: "deb https://dl.bintray.com/sbt/debian /"
+        key_url: "https://bintray.com/user/downloadSubjectPublicKey?username=sbt"
+    packages:
+      - sbt=1.2.7
+
 before_install:
   - PATH=$(echo "$PATH" | sed -e 's/:\/usr\/local\/lib\/jvm\/openjdk11\/bin//')
   - JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-amd64