Run "gem update" to avoid Gem::Ext::BuildError

The following error occurs when installing rainbow gem:

```
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    can't modify frozen String
```
ref. ku1ik/rainbow#49

Author: yasaichi

.travis.yml

@@ -18,5 +18,7 @@ matrix:
 branches:
   only:
     - master
+before_install:
+  - 'gem update --system --no-doc'
 after_script:
   - '[ ${TRAVIS_EVENT_TYPE} != "pull_request" ] && [ ${TRAVIS_BRANCH} = "master" ] && bundle exec codeclimate-test-reporter'