[Travis] make `.bundle` directory writable

To create gem files from sources under `gems/src` for tests.

.travis.yml

@@ -173,7 +173,7 @@ before_script:
     fi
   - mkdir config_1st config_2nd gems/src
   - chmod -R a-w .
-  - chmod -R u+w build config_1st config_2nd gems/src
+  - chmod -R u+w build config_1st config_2nd gems/src .bundle
   - cd build
   - |-
     case "$CC" in