Minimum language level for reporters

[codefromthecrypt]

In Brave, the minimum language level for core code is 1.6 as there are a number of legacy apps and/or agents that cannot move ahead of that. This is also the case in zipkin-reporter. The minimum level for collectors is java 7, as custom servers needn't be so low.

There are libraries which have higher language level, such as okhttp (java7) and I'm not sure the minimum level for AWS sdk (haven't looked).

Whatever we decide here is important here, and should be noted on the README.