Removes oraclejdk7 from Travis since it is not supported.

Message from development list.

oraclejdk7 is not supported on Trusty image.
travis-ci/travis-ci#7884

In the core, I just specified the old Precise image as a temporary
workaround.
mybatis/mybatis-3@5f787f1

...but I think it's OK to remove oraclejdk7 from the target JDK list
(openjdk7 works fine on Trusty).

.travis.yml

@@ -3,7 +3,6 @@ sudo: false
 
 jdk:
   - oraclejdk8
-  - oraclejdk7
   - openjdk7
 
 after_success: