Speed up Travis CI build

[qinzuoyan]

now building pegasus takes more than 30 minutes (travis ci timeout is 50 minutes), we need to speed up it.

[qinzuoyan]

some efforts: #83
but not enough

[qinzuoyan]

Refer to: https://eng.localytics.com/best-practices-and-common-mistakes-with-travis-ci/
"By default, Travis will build twice for each pull request. One of the builds will be the build for the branch itself, and one of the builds will be for the potential future merge commit against the target of the pull request. For us, because we merge to master quickly, it’s rare that the merge commit build will find something that the branch build doesn’t. Because of that, we’ve disabled the second build for a lot of our less fundamental services."

We choose to build only pushed pull requests.

[qinzuoyan]

more efforts: #85