Added conf.enable_test

[inokappa]

HI, matsumoto-san.

I tried to create sample mrbgem refer to following blog post.

• http://hb.matsumoto-r.jp/entry/2015/10/31/202645
• http://blog.matsumoto-r.jp/?p=3923

The test of sample mrbgem was not running when I pushed it into github.(refer to following)

• https://travis-ci.org/inokappa/mruby-sample/jobs/88592351

I found It was solved by adding conf.enable_test to .travis_build_config.rb.
Therefore I modified so that conf.enable_test was defined in .travis_build_config.rb.

Please confirm.

Thank you.

[matsumotory]

👍