set PR additions file as link validation file

build/main.sh

 #!/bin/bash
-CHECK_FILE=../README.md
+FORMAT_FILE=../README.md
+if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+    echo "running on Pull Request #$TRAVIS_PULL_REQUEST"
+    git show | egrep "\+" > additions.txt
+    LINK_FILE=additions.txt
+else
+    echo "running on $TRAVIS_BRANCH branch"
+    LINK_FILE=../README.md
+fi
+
 echo "running format validation..."
-./validate_format.rb $CHECK_FILE
+./validate_format.rb $FORMAT_FILE
 if [[ $? != 0 ]]; then
     echo "format validation failed!"
     exit 1
@@ -11,7 +20,7 @@ fi
 
 if [ "$TRAVIS_BRANCH" == "master" ]; then
     echo "running link validation..."
-    ./validate_links.rb $CHECK_FILE
+    ./validate_links.rb $LINK_FILE
     if [[ $? != 0 ]]; then
         echo "link validation failed!"
         exit 1