Drop support for Java 7

[szpak]

Java 7 has been EOLed years ago, it's not easily accessible from Oracle, it is deprecated in Gradle 4 and there are issues in using it on Travis.

• - display warning for Java 7 users
• - document ability to build with Java 7 while running Gradle with Java 8
• - switch artifacts compatibility to Java 8

[szpak]

Newer Gradle 4 versions display deprecation warning about Java 7 which breaks the functional tests (which check for deprecated stuff).

[szpak]

Issues with Java 7:

• deprecated as of April 2016
• deprecated in Gradle 4
• issues issues related to maintaining of regression test harness with different Java versions (some Gradle/PIT version are not Java 9 compatible, some Gradle version doesn't like Java 7 builds)
• limited availability of Java 7 on CI servers

The Java 7 support has been terminated as of gradle-pitest-plugin 1.3.0 (as a JDK version required to run a Gradle build). The latest version with Java 7 compatible binaries is 1.2.4.