ISSUE #177: mvn release failed due to wrong scm url

Descriptions of the changes in this PR:

Reason:

we are using `gitbox` url as the scm connection. However gitbox is a readonly mirror of github repo.
The committers don't have permissions to commit directly to gitbox repo.

Solution:

update the scm connection to github url

Author: Sijie Guo <sijie@apache.org>

Reviewers: Jia Zhai <None>

This closes #178 from sijie/fix_scm_url, closes #177
diff --git a/pom.xml b/pom.xml
index 4d8e44d..3c19a5d 100644
--- a/pom.xml
+++ b/pom.xml
@@ -40,10 +40,10 @@
     </license>
   </licenses>
   <scm>
-    <connection>scm:git:https://gitbox.apache.org/repos/asf/distributedlog.git</connection>
-    <developerConnection>scm:git:https://gitbox.apache.org/repos/asf/distributedlog.git</developerConnection>
-    <url>https://gitbox.apache.org/repos/asf?p=distributedlog.git;a=summary</url>
-    <tag>release-0.5.0</tag>
+    <connection>scm:git:https://github.com/apache/distributedlog.git</connection>
+    <developerConnection>scm:git:https://github.com/apache/distributedlog.git</developerConnection>
+    <url>https://github.com/apache/distributedlog</url>
+    <tag>HEAD</tag>
   </scm>
   <issueManagement>
     <system>jira</system>