build(SdkCommon): Add checkstyle to common class (and clean up subsequent complaint)

Author: lpatino10

common/build.gradle

@@ -1,7 +1,13 @@
 import org.apache.tools.ant.filters.*
 
+apply plugin: 'checkstyle'
 apply plugin: 'java'
 
+checkstyle {
+    configFile = rootProject.file('checkstyle.xml')
+    ignoreFailures = false
+}
+
 repositories {
     mavenCentral()
 }

common/src/main/java/com/ibm/watson/common/SdkCommon.java

@@ -14,6 +14,8 @@ public class SdkCommon {
   private static final Logger LOG = Logger.getLogger(SdkCommon.class.getName());
   private static String userAgent;
 
+  private SdkCommon() { }
+
   private static String loadSdkVersion() {
     ClassLoader classLoader = SdkCommon.class.getClassLoader();
     InputStream inputStream = classLoader.getResourceAsStream("java-sdk-version.properties");