Update Maven core- and third-party plugin versions

* Update "maven-compiler-plugin" to version 3.6.1
  Starting with version 3.5, this plugin requires Maven 3. This version
  adds support for Java 9.
* Update "jacoco-maven-plugin" to version 0.7.9
  Next to a bunch of important bug fixes and a new user property for the
  data file the compatibility for Java 9 has been improved.
* Update "nexus-staging-maven-plugin" to version 1.6.8
  The the release version is compatible with the latest Nexus client lib.

Also removed the unused test dependency "org.hamcrest:hamcrest-library".

GH-17

References:
https://issues.apache.org/jira/secure/ReleaseNote.jspa?projectId=12317225&version=12338627

icecore-hashids.iml

@@ -14,6 +14,5 @@
     <orderEntry type="sourceFolder" forTests="false" />
     <orderEntry type="library" scope="TEST" name="Maven: junit:junit:4.12" level="project" />
     <orderEntry type="library" scope="TEST" name="Maven: org.hamcrest:hamcrest-core:1.3" level="project" />
-    <orderEntry type="library" scope="TEST" name="Maven: org.hamcrest:hamcrest-library:1.3" level="project" />
   </component>
 </module>

pom.xml

@@ -70,7 +70,10 @@
     <assembly.descriptor.bin>${basedir}/src/main/assets/assembly-bin.xml</assembly.descriptor.bin>
     <java.version>1.8</java.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+
     <checkstyle.version>8.0</checkstyle.version>
+    <junit.version>4.12</junit.version>
+    <hamcrest.version>1.3</hamcrest.version>
   </properties>
 
   <scm>
@@ -89,21 +92,14 @@
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
-      <version>4.12</version>
+      <version>${junit.version}</version>
       <scope>test</scope>
     </dependency>
 
     <dependency>
       <groupId>org.hamcrest</groupId>
       <artifactId>hamcrest-core</artifactId>
-      <version>1.3</version>
-      <scope>test</scope>
-    </dependency>
-
-    <dependency>
-      <groupId>org.hamcrest</groupId>
-      <artifactId>hamcrest-library</artifactId>
-      <version>1.3</version>
+      <version>${hamcrest.version}</version>
       <scope>test</scope>
     </dependency>
   </dependencies>
@@ -114,7 +110,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.5.1</version>
+        <version>3.6.1</version>
         <configuration>
           <source>${java.version}</source>
           <target>${java.version}</target>
@@ -163,7 +159,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
-        <version>2.10.4</version>
+        <version>3.0.0-M1</version>
         <executions>
           <execution>
             <id>attach-javadocs</id>
@@ -203,6 +199,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-checkstyle-plugin</artifactId>
+        <version>2.17</version>
         <dependencies>
           <dependency>
             <groupId>com.puppycrawl.tools</groupId>
@@ -271,7 +268,7 @@
           <plugin>
             <groupId>org.jacoco</groupId>
             <artifactId>jacoco-maven-plugin</artifactId>
-            <version>0.7.7.201606060606</version>
+            <version>0.7.9</version>
             <executions>
               <execution>
                 <goals>
@@ -330,7 +327,7 @@
           <plugin>
             <groupId>org.sonatype.plugins</groupId>
             <artifactId>nexus-staging-maven-plugin</artifactId>
-            <version>1.6.7</version>
+            <version>1.6.8</version>
             <extensions>true</extensions>
             <configuration>
               <serverId>ossrh</serverId>