[TravisCI] Make gems/src directory writable

.travis.yml

@@ -170,9 +170,9 @@ before_script:
     else
       mkdir build
     fi
-  - mkdir config_1st config_2nd
+  - mkdir config_1st config_2nd gems/src
   - chmod -R a-w .
-  - chmod -R u+w build config_1st config_2nd
+  - chmod -R u+w build config_1st config_2nd gems/src
   - cd build
   - |-
     case "$CC" in