Exclude greenkeeper merges from changelog

There is a great deal of lines about Greenkeeper merges in the changelog, making it
hard to find real changes to the code.

This commit modifies the changelog task to exclude such lines.

tasks/changelog.sh

@@ -44,6 +44,7 @@ main() {
   git log --first-parent --format='%aN|%s %b' ${1} |
   {
     while read l; do
+      if ! [[ ${l} =~ "openlayers/greenkeeper" ]] ; then
         if [[ ${l} =~ ${MERGE_RE} ]] ; then
           number="${BASH_REMATCH[1]}"
           author="${BASH_REMATCH[2]}"
@@ -55,6 +56,7 @@ main() {
           summary="${BASH_REMATCH[2]}"
           echo " * [#${number}](${PULLS_URL}/${number}) - ${summary} ([${author}](${GITHUB_URL}/search?q=${author}&type=Users))"
         fi
+      fi
     done
   }
 }