Use build status image for master branch only

The build status image has a query string that can be used to
specify the branch name for which the build status should be
displayed. Add query string to the URL to the build status so
that build status of the master branch is displayed.
Install coveralls before running build script

The first Travis CI build failed because coveralls is not
installed before submitting code coverage data to coveralls.io.
Fix this issue by installing coveralls before running the build
script.