[REEF-1858] Pin the Travis tests to Ubuntu `precise`

Added `dist: precise` to `.travis.yml` to stay on the older Ubuntu release.

JIRA:
  [REEF-1858](https://issues.apache.org/jira/browse/REEF-1858)

Pull Request:
  This closes #1358
diff --git a/.travis.yml b/.travis.yml
index fe414a4..e460051 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,6 +17,8 @@
 
 language: java
 
+dist: precise
+
 jdk:
   - oraclejdk7
   - oraclejdk8