reduced signature dependency to Java 1.6
reduced signature dependency to Java 1.6

census-instrumentation/opencensus-java,sebright/opencensus-java,songy23/instrumentation-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,bogdandrutu/instrumentation-java,bogdandrutu/instrumentation-java,bogdandrutu/opencensus-java,sebright/opencensus-java,census-instrumentation/opencensus-java,sebright/instrumentation-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,sebright/opencensus-java,census-instrumentation/opencensus-java,sebright/instrumentation-java