add retrying of a Runnable

[qct]

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[qct]

I got the same problem like #64 . Dan Lew doesn't suggest the dynamic versions for dependencies.

check this: Don't use dynamic versions for your dependencies

I think guava-retrying should release a new version like 3.0 or something that only support jdk 1.8. So guava-retrying can depends on a fixed version of guava, like 21.0.

leave the 2.0 version depends on JDK 1.6+， and a fixed version of guava, like 10.0 or 20.0.