Specify Ubuntu Precise only for Java 7 Travis build job.
Specify Ubuntu Precise only for Java 7 Travis build job.

The other build jobs can upgrade to the new Travis default of Ubuntu Trusty,
because the Java 7 build was the only failure (#475).

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