Drop support for Java 1.6

[marek-parfianowicz]

Language level and runtime.

Currently we must compile using 1.6 source level. It it problematic as e.g. JUnit5 requires Java 8. Drop it as Java 1.6 usage is marginal. Tasks:

• bump source/target levels to 1.7 (ant / maven / gradle)
• update developer guides on the website (java 7/8 required)
• update dev guide about recommended maven (we can use 3.6.3 now - previously recommended was 3.2.5 due to compatibility with java 6)
• get rid of the java_rt-1.6.jar from the build

[marek-parfianowicz]

Original report: https://bitbucket.org/openclover/clover/issues/101

[marek-parfianowicz]

Code changes:

• clover - https://github.com/openclover/clover/pull/118/commits
• clover-aspectj-compiler - openclover/clover-aspectj-compiler@043534f
• clover-examples - openclover/clover-examples@6369865
• clover-maven-plugin - openclover/clover-maven-plugin@49cf96b
• gradle-clover-plugin - openclover/gradle-clover-plugin@7b2d82b
• jenkins-clover-plugin - jenkinsci/clover-plugin@a02d34e

[marek-parfianowicz]

Developer guides are for 4.5.0 version.