Added download section for the next release

It is properly the current release since it was accidentally forgotten
to update the download page.
Updated SLF4J to version 1.7.5 and Logback to version 1.0.11
Updated Maven Surefire Plugin to 2.14.1
Removed useless parenthesis
Updated Maven PMD Plugin to 3.0.1
Changed fields to 'final' in example code
Added JSR-305 Annotations, a transitive dependency of Quality-Check, as
managed dependency
Added JSR-305 Annotations and replaced custom argument checks with
Quality-Check's alternatives
Switched from Cobertura to JaCoCo to measure code coverage
Removed unused import
Removed redundant null-check of VersionNumber.getGroups(), because the
interface defines it as non-nullable
Corrected JSR-305 Annotations on fields
Added check that the existing cache file will be deleted if it exists
Added check that no operating system builder with the same ID already
exists
Corrected type of @throws documentation
Updated requirements section to regard now compile and runtime
dependencies
Updated wording to be clearer