Always use Homebrew Ruby

Travis reports the following error even rvm uses 2.3.x:

```
/usr/lib/ruby/1.9.1/rubygems/version.rb:191:in `strip!': can't modify frozen String (RuntimeError)
	from /usr/lib/ruby/1.9.1/rubygems/version.rb:191:in `initialize'
	from -e:1:in `new'
	from -e:1:in `<main>'
/home/travis/.linuxbrew/Library/Homebrew/brew.rb:12:in `<main>': Homebrew must be run under Ruby 2.3! You're running 1.9.3. (RuntimeError)
```

Force to use Homebrew vendored ruby to solve this issue.

[1]: Linuxbrew/brew@40af9e6#diff-354f30a63fb0907d4ad57269548329e3
[2]: Homebrew/brew@40af9e6#diff-354f30a63fb0907d4ad57269548329e3
[3]: https://github.com/Linuxbrew/brew/blob/02773af31860e14d247def718a25b16667e94012/Library/Homebrew/cmd/vendor-install.sh
[4]: https://github.com/Homebrew/brew/blob/75d032129a94210937718ac72c5e74a59360a5fd/Library/Homebrew/cmd/vendor-install.sh
[5]: Homebrew/brew#3240
[6]: Homebrew/brew#3546

Author: dochang

.travis.yml

@@ -1,6 +1,6 @@
 dist: trusty
 language: ruby
-rvm: 2.0.0
+rvm: system
 sudo: false
 
 addons:
@@ -17,6 +17,7 @@ env:
     - LINUXBREW=$HOME/.linuxbrew
     - HOMEBREW_DEVELOPER=1
     - HOMEBREW_NO_AUTO_UPDATE=1
+    - HOMEBREW_FORCE_VENDOR_RUBY=1
     - secure: "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"
 
 before_install: