Use master branch status only for Travis CI badges

Problem

Without a branch parameter in the image URL, the Travis CI badge will show the
status of the latest build. We don't guarantee that develop branches will
compile without local publication of dependencies, which means the badge may not
always be accurate.

Solution

Add a branch parameter so that we only show the build status of master.

RB_ID=544318
Use Travis CI's new container-based infrastructure

Problem

Travis CI just announced a new container-based infrastructure that provides
faster builds, etc., and can be enabled by disabling sudo in the Travis config.

Solution

We don't need sudo for any of these projects, so we can disable it. I've also
fixed a few minor issues with versions in the Travis configuration for scrooge,
finagle, and twitter-server.

RB_ID=547354