ci: Add Pull request check to semaphore scripts

This fixes the conditions of build failure when someone created a
pull request from the master branch. It now also checks if it is a
Pull request or not.

Fixes coala#583

.ci/semaphore.answers.sh

@@ -5,7 +5,7 @@ pull_and_build() {
     docker build -t meetmangukiya/corobo-answers answers/
 }
 
-if [[ $BRANCH_NAME != "master" ]]
+if [[ $BRANCH_NAME != "master" && -n $PULL_REQUEST_NUMBER ]]
 then
     .ci/check_docker.py
     if [[ $? == 1 ]]

.ci/semaphore.docker.sh

@@ -12,6 +12,6 @@ docker run --user root meetmangukiya/corobo /bin/sh -c "
     python -m pytest
 "
 
-if [[ $BRANCH_NAME == "master" ]]
+if [[ $BRANCH_NAME == "master" && -z $PULL_REQUEST_NUMBER ]]
 then echo "pushing..." && docker push meetmangukiya/corobo && docker push meetmangukiya/corobo-answers
 fi