[LIVY-557] Make Travis-CI logs less verbose
[LIVY-557] Make Travis-CI logs less verbose

## What changes were proposed in this pull request?
- Set the base travis image to ubuntu Xenial.
- Set the Scala version to `2.11.12` to match the version in pom.xml.
- Install `r-base` directly from xenial repos (no need to add r-packages).
- Combine the two `mvn` builds into one (`script` and `install` sections in `.travis.yml`).
- Make apt-get and pip are quiet.
- ~Set the SLF4J log level to warn for maven.~

## How was this patch tested?
Tested on Travis:
- Before this change, travis logs were about 6K+ lines long.
- After this change, the logs are less than 4K for unit tests and less than 3K for integration tests.
- I also verified that
  - Travis uses scala 2.11.12.
  - r-base is installed.
  - mvn runs without a problem after combining the two mvn builds.
  - Warn level messages and outputs of builds/tests are still available in the logs.
  - apt-get and pip run in quiet mode.

## Sample Travis outputs
**A build log that is  big to fit into Travis UI (size 4MB)**: https://travis-ci.org/apache/incubator-livy/jobs/487096239
**A build log BEFORE this change  (size 643KB)**: https://travis-ci.org/apache/incubator-livy/jobs/487164276
**A build log AFTER this change (size 327KB)**: https://travis-ci.org/apache/incubator-livy/jobs/492011690
**~A build log with mvn log level set to warn (323 KB)**: ~https://travis-ci.org/apache/incubator-livy/jobs/490937880~
This change is reverted because it was silencing useful information in mvn.

Task-url: https://issues.apache.org/jira/browse/LIVY-557

Author: Fathi Salmi, Meisam(mfathisalmi) <mfathisalmi@paypal.com>
Author: Fathi Salmi, Meisam <meisam.fathi@gmail.com>
Author: Meisam Fathi <meisam.fathi@gmail.com>

Closes #144 from meisam/LIVY-557.

meisam/incubator-livy,meisam/incubator-livy,ajbozarth/incubator-livy,ajbozarth/incubator-livy,meisam/incubator-livy,ajbozarth/incubator-livy,ajbozarth/incubator-livy,ajbozarth/incubator-livy,meisam/incubator-livy,meisam/incubator-livy,meisam/incubator-livy,ajbozarth/incubator-livy