Fix Travis build

The maven-gpg-plugin broke the Travis build with an error

> gpg: no default secret key: secret key not available

Fixed it by moving the GPG plugin and other release-related
plugins to the release profile which is not used by Travis.

pom.xml

@@ -71,6 +71,14 @@
           <compilerArgument>-Xlint:all</compilerArgument>
         </configuration>
       </plugin>
+    </plugins>
+  </build>
+
+  <profiles>
+    <profile>
+      <id>release</id>
+      <build>
+        <plugins>
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-source-plugin</artifactId>
@@ -116,4 +124,6 @@
           </plugin>
         </plugins>
       </build>
+    </profile>
+  </profiles>
 </project>