Add project.url to satisfy the dependency-check-maven plugin

The MANIFEST.MF contains an Implementation-URL key, that comes from
project.url if defined. If not defined, jjwt's url comes from sonatype
which seems to falsely associate it with a sonatype/nexus CVE and
fails the project build.

#234

Author: Mark Kempster

pom.xml

@@ -28,6 +28,7 @@
     <version>0.10.0-SNAPSHOT</version>
     <name>JSON Web Token support for the JVM</name>
     <packaging>jar</packaging>
+    <url>https://github.com/jwtk/jjwt</url>
 
     <licenses>
         <license>