yaml use lower boolean strings

Author: yetist

.travis.yml

@@ -208,12 +208,12 @@ after_scripts:
   - make distcheck
 
 releases:
-  draft: False
-  prerelease: False
-  checksum: True
-  file_glob: True
+  draft: false
+  prerelease: false
+  checksum: true
+  file_glob: true
   files: mate-control-center-*.tar.xz
   github_release:
-    tags: True
-    overwrite: True
+    tags: true
+    overwrite: true
     base_version: 1.20.0