Remove Support for Java 7

Java 7 has long reached the end of its life support, yet Tephra still
supports and tests with Java 7. After the recent change to use HTTPS, Java
7 causes repeated test failures, caused by its lack of support for TLS 1.2 (
https://central.sonatype.org/articles/2018/May/04/discontinued-support-for-tlsv11-and-below/)