[SPARK-19801][BUILD] Remove JDK7 from Travis CI

## What changes were proposed in this pull request?

Since Spark 2.1.0, Travis CI was supported by SPARK-15207 for automated PR verification (JDK7/JDK8 maven compilation and Java Linter) and contributors can see the additional result via their Travis CI dashboard (or PC).

This PR aims to make `.travis.yml` up-to-date by removing JDK7 which was removed via SPARK-19550.

## How was this patch tested?

See the result via Travis CI.

- https://travis-ci.org/dongjoon-hyun/spark/builds/207111713

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #17143 from dongjoon-hyun/SPARK-19801.

.travis.yml

@@ -28,7 +28,6 @@ dist: trusty
 # 2. Choose language and target JDKs for parallel builds.
 language: java
 jdk:
-  - oraclejdk7
   - oraclejdk8
 
 # 3. Setup cache directory for SBT and Maven.