use set instead of map for ttl instances holder by Collections.newSetFromMap
https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#newSetFromMap-java.util.Map-

Merge pull request #95 from fxleyu/master

codeconv-bash upload codecov report of tag to master branch

- https://github.com/codecov/codecov-python
- https://github.com/codecov/codecov-bash

- https://docs.codecov.io/docs
- https://github.com/codecov/example-java-maven
- https://github.com/codecov/example-kotlin

- build by java 8 with java 6 source/target
    - full functions with java 8 support(ForkJoinPoolTest)
    - support java 6
- Travis ci has multiple java version: java 6/7/8/9
- scripts and test case/demo cleanup