allow spec build on jdk 12+

Signed-off-by: Lukas Jungmann <lukas.jungmann@oracle.com>

spec/pom.xml

diff --git a/spec/pom.xml b/spec/pom.xml
index d7b5a4b..75e3cea 100644
--- a/spec/pom.xml
+++ b/spec/pom.xml
@@ -35,9 +35,7 @@
         <site.output.dir>${project.build.directory}/staging</site.output.dir>
         <maven.site.skip>true</maven.site.skip>
         <asciidoctor.maven.plugin.version>1.5.7.1</asciidoctor.maven.plugin.version>
-        <asciidoctorj.version>1.6.2</asciidoctorj.version>
         <asciidoctorj.pdf.version>1.5.0-alpha.16</asciidoctorj.pdf.version>
-        <jruby.version>9.2.6.0</jruby.version>
         <!-- status: DRAFT, BETA, etc., or blank for final -->
         <status>DRAFT</status>
         <maven.build.timestamp.format>MMMM dd, yyyy</maven.build.timestamp.format>
@@ -63,7 +61,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-enforcer-plugin</artifactId>
-                <version>1.4.1</version>
+                <version>3.0.0-M3</version>
                 <executions>
                     <execution>
                         <id>enforce-versions</id>
@@ -73,7 +71,7 @@
                         <configuration>
                             <rules>
                                 <requireJavaVersion>
-                                    <version>(,1.9),[11,)</version>
+                                    <version>[8,)</version>
                                     <message>You need JDK8 or newer</message>
                                 </requireJavaVersion>
                             </rules>
@@ -87,16 +85,6 @@
                 <version>${asciidoctor.maven.plugin.version}</version>
                 <dependencies>
                     <dependency>
-                        <groupId>org.jruby</groupId>
-                        <artifactId>jruby-complete</artifactId>
-                        <version>${jruby.version}</version>
-                    </dependency>
-                    <dependency>
-                        <groupId>org.asciidoctor</groupId>
-                        <artifactId>asciidoctorj</artifactId>
-                        <version>${asciidoctorj.version}</version>
-                    </dependency>
-                    <dependency>
                         <groupId>org.asciidoctor</groupId>
                         <artifactId>asciidoctorj-pdf</artifactId>
                         <version>${asciidoctorj.pdf.version}</version>
@@ -165,31 +153,13 @@
                 </configuration>
 
             </plugin>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-release-plugin</artifactId>
-                <version>2.5.2</version>
-                <configuration>
-                    <mavenExecutorId>forked-path</mavenExecutorId>
-                    <useReleaseProfile>false</useReleaseProfile>
-                    <arguments>${release.arguments}</arguments>
-                </configuration>
-                <dependencies>
-                    <dependency>
-                        <groupId>org.apache.maven.scm</groupId>
-                        <artifactId>maven-scm-provider-gitexe</artifactId>
-                        <version>1.9.4</version>
-                    </dependency>
-                </dependencies>
-            </plugin>
-
             <!--
                 This is the rule that builds the zip file for download.
             -->
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-assembly-plugin</artifactId>
-                <version>3.1.1</version>
+                <version>3.2.0</version>
                 <inherited>false</inherited>
                 <executions>
                     <execution>