Disable travis ci on linux

The travis ci on linux seems to be unstable than on mac, so try to disable travis ci on linux image now.

Author: jiazhai <zhaijia03@gmail.com>

Reviewers: Sijie Guo <sijie@apache.org>

Closes #26 from jiazhai/zhaijia/disable_linux_ci
diff --git a/.travis.yml b/.travis.yml
index 74ff391..e5ee4db 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,8 +17,6 @@
 
 matrix:
   include:
-    - os: linux
-      jdk: oraclejdk8
     - os: osx
       osx_image: xcode8