Replace the java 1.8 only dx.jar with one supporting java 1.7

Also removed the travis test for java 1.6.

Java 1.7 version of dx.jar compiled with plaform/libcore at commit
c3e562a and platform/dalvik at commit db9197b with
https://android-review.googlesource.com/#/c/221127/1 cherry-picked on
top of it.

.travis.yml

@@ -2,7 +2,6 @@ language: java
 jdk:
     - oraclejdk8
     - oraclejdk7
-    - openjdk6
 
 before_install:
     - chmod +x gradlew