Revert &. auto corrections and use Ruby v2.0 for Rubocop

The save navigation operator (&.) is supported since Ruby v2.3.0
(JRuby v9.1.0.0, see jruby/jruby#3479).
So these autocorrection would fail for lower versions, like 9.0.5.0,
which is e.g. used by Travis CI.

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.4
+  TargetRubyVersion: 2.0
   Exclude:
     - '*.gemspec'
     - 'Gemfile'

lib/jar_dependencies.rb

@@ -187,7 +187,7 @@ def home
 
     def require_jars_lock!(scope = :runtime)
       urls = jars_lock_from_class_loader
-      if !urls&.empty?
+      if urls && !urls.empty?
         @@jars_lock = true
         # funny error during spec where it tries to load it again
         # and finds it as gem instead of the LOAD_PATH

lib/jars/installer.rb

@@ -68,7 +68,7 @@ def self.load_from_maven(file)
       result = []
       File.read(file).each_line do |line|
         dep = Dependency.new(line)
-        result << dep if dep&.scope == :runtime
+        result << dep if dep && dep.scope == :runtime
       end
       result
     end
@@ -101,12 +101,12 @@ def self.write_dep(file, _dir, dep, _vendor)
     def self.print_require_jar(file, dep, fallback = false)
       return if dep.type != :jar || dep.scope != :runtime
       if dep.system?
-        file&.puts("require '#{dep.file}'")
+        file.puts("require '#{dep.file}'") if file
       elsif dep.scope == :runtime
         if fallback
-          file&.puts("  require '#{dep.path}'")
+          file.puts("  require '#{dep.path}'") if file
         else
-          file&.puts("  require_jar '#{dep.gav.gsub(':', "', '")}'")
+          file.puts("  require_jar '#{dep.gav.gsub(':', "', '")}'") if file
         end
       end
     end

specs/jars_spec.rb

@@ -57,7 +57,7 @@
 
   it 'extract maven settings' do
     settings = Jars.maven_settings
-    settings&.sub(/.*\.m2./, '')&.must_equal 'settings.xml'
+    settings.sub(/.*\.m2./, '').must_equal 'settings.xml' if settings
 
     ENV['JARS_MAVEN_SETTINGS'] = 'specs/settings.xml'
     Jars.reset