add and configure coveralls-maven-plugin

In order to report code coverage to Coveralls.io add the
`coveralls-maven-plugin` [1] and fix the error occured on
execution

> [ERROR] Failed to execute goal org.eluder.coveralls:coveralls-maven-plugin:4.3.0:report (default-cli) on project org.everit.json.schema: Build error: Source encoding not set, use <sourceEncoding> configuration option or set project wide property <project.build.sourceEncoding> -> [Help 1]

simply by adding both both configuration properties
* `<project.build.sourceEncoding/>`
* `<project.reporting.outputEncoding/>`

[1] https://github.com/trautonen/coveralls-maven-plugin

Author: agebhar1

core/pom.xml

@@ -24,6 +24,8 @@
     <version>1.4.1</version>
     <packaging>bundle</packaging>
     <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
         <projectpath>json-schema</projectpath>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
@@ -96,6 +98,11 @@
                     <argLine>${surefireArgLine}</argLine>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.eluder.coveralls</groupId>
+                <artifactId>coveralls-maven-plugin</artifactId>
+                <version>4.3.0</version>
+            </plugin>
         </plugins>
     </build>
     <dependencies>