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

[dongjoon-hyun]

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

[dongjoon-hyun]

Thank you, @srowen .

[SparkQA]

Test build #73785 has finished for PR 17143 at commit d3ab0e9.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

The only failure is irrelevant to this PR.

[info] KafkaSourceStressForDontFailOnDataLossSuite:
[info] - stress test for failOnDataLoss=false *** FAILED *** (1 minute, 2 seconds)
[info]   org.apache.spark.sql.streaming.StreamingQueryException: Query [id = 799c24be-7e4f-4fec-a24c-a3b4a98c676a, runId = 533f402f-e589-4c7b-81e1-7b8c064b603c] terminated with exception: java.lang.InterruptedException

[dongjoon-hyun]

Retest this please

[SparkQA]

Test build #73788 has finished for PR 17143 at commit d3ab0e9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Thank you, @felixcheung .

[srowen]

Merged to master

[dongjoon-hyun]

Thank you for merging, @srowen .