Upgrade to Checkstyle 6.13

Breaking backward compatibility:

    Remove access to undocumented check properties. Author: rnveach #2451

New:

    ParameterName: new option to skip methods with Override annotation. Author: Andrei Selkin #2290

Bug fixes:

    False negative in RequireThis check. Author: Vladislav Lisetskiy #2240
    Checkstyle throws NumberFormatException on Japanese locale when sizes and metrics Checks are used. Author: Takahashi Eikou #2601
    EmptyLineSeparator check does not validate newlines before class and after last method. Author: Vladislav Lisetskiy #2067
    FallThroughCheck handles finally incorrectly. Author: Vladislav Lisetskiy #1438
    EqualsAvoidNull check should сheck String concatenations. Author: Vladislav Lisetskiy #1930

Notes:

    minor: remove maven-failsafe-plugin from linkcheck excludes, since link was fixed. Author: Andrei Selkin
    minor: build-helper-maven-plugin version bump to 1.10. Author: Roman Ivanov
    adjust AbbreviationAsWordInName in checkstyle_checks.xml to catch unexpected abbreviations. Author: Roman Ivanov #2639
    Fix additional issues reported by IntelliJ IDEA inspections in Checkstyle code. Author: Michal Kordas #2080
    minor: fixed improper put that should be get. Author: rnveach
    Code review of CommentsIndentationCheck.java. Author: Andrei Selkin #2530
    Remove drag&drop from GUI. Author: Ilja Dubinin #2608
    create HTML web page to describe how to open issues against Checkstyle. Author: Michal Kordas, Roman Ivanov #2344
    Pull #2607: Update JGit to 4.1.1.201511131810-r. Author: Michal Kordas #2607
    Pull #2605: Make methods private in RequireThisCheck. Author: Vladislav Lisetskiy #2605
    Tests for validating commit message should not allow period at the end of commit message. Author: Andrei Selkin #2590
    doc: Correct xdoc/javadoc for ParameterNameCheck. Author: Andrei Selkin
    unify IT test code. Author: rnveach #2589
    minor: separated tests so one subject per file. Author: rnveach
    Deactivate 'allowSamelineSingleParameterlessAnnotation' property of AnnotationLocation in Checkstyle config. Author: Michal Kordas #2581
    Create test for checkstyle-X.X-all.jar on Travis. Author: Roman Ivanov #2516
    Pull #2586: Update slf4j-simple to 1.7.13. Author: Michal Kordas #2586
    Pull #2585: Update FindBugs Maven Plugin to 3.0.3. Author: Michal Kordas #2585
    PMD: resolve problems from DefaultPackage rule. Author: Michal Kordas #957
    minor: fixed wrong variable usage in AbstractOptionCheck. Author: rnveach
    Remove usage of System.out.println in IT resources. Author: Michal Kordas, Roman Ivanov #2541
    Unify naming of all files with IT inputs. Author: rnveach #2540
    UT to validate google/sun style xdocs. Author: rnveach #2557
    Unify naming of all files with test inputs. Author: rnveach, Roman Ivanov #2161
    spelling: fix strange japanese.. Author: Nobuyuki-Inaba
    Improve documentation for CovariantEquals check. Author: Vladislav Lisetskiy #1031