Don't include branch name in Knapsack report file

Otherwise, branches other than master won't benefit from Knapsack!
Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab-ci.yml

@@ -14,8 +14,8 @@ variables:
   GIT_DEPTH: "20"
   PHANTOMJS_VERSION: "2.1.1"
   GET_SOURCES_ATTEMPTS: "3"
-  KNAPSACK_RSPEC_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/rspec_report-${CI_COMMIT_REF_SLUG}.json
-  KNAPSACK_SPINACH_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/spinach_report-${CI_COMMIT_REF_SLUG}.json
+  KNAPSACK_RSPEC_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/rspec_report-master.json
+  KNAPSACK_SPINACH_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/spinach_report-master.json
 
 before_script:
   - source ./scripts/prepare_build.sh