chore(.travis.yml): add 'travis_wait' to doxygen build

"If you have a command that doesn't produce output for
more than 10 minutes, you can prefix it with travis_wait,
 a function that's exported by our build environment."

http://docs.travis-ci.com/user/build-timeouts/
Merge pull request #13 from soonhokong/master

chore(.travis.yml): add 'travis_wait' to doxygen build