use the official sbt script

This is a workaround for Travis CI is using an old cut of sbt-extras, which prevents the use of sbt 1 published on Maven Central.

Ref travis-ci/travis-ci#9816

.travis.yml

@@ -44,6 +44,18 @@ env:
     - SBT_CMD="scripted tests/*"
     - SBT_CMD="repoOverrideTest:scripted dependency-management/*"
 
+before_install:
+  - export SBT_OPTS="-Dsbt.log.noformat=true"
+  - export PATH=/usr/bin:$PATH
+
+addons:
+  apt:
+    sources:
+      - sourceline: 'deb https://dl.bintray.com/sbt/debian /'
+        key_url: 'https://bintray.com/user/downloadSubjectPublicKey?username=sbt'
+    packages:
+      - sbt=1.1.6
+
 notifications:
   email:
     - sbt-dev-bot@googlegroups.com