Revert back to using Travis SBT

Since it's already the same as what we are using (sbt-extras).
See: https://github.com/travis-ci/travis-ci/issues/3140

.travis.yml

@@ -7,7 +7,7 @@ jdk:
     - oraclejdk8
 
 script:
-    - cd $TRAVIS_BUILD_DIR && ./sbt ++$TRAVIS_SCALA_VERSION clean coverage test it:test
+    - sbt ++$TRAVIS_SCALA_VERSION clean coverage test it:test
 
 after_success:
-    - cd $TRAVIS_BUILD_DIR && ./sbt coveralls
+    - sbt coveralls