manage dependencies with godep

This commit adds all cadvisor dependencies as the result of running
the following commands:

    go get -u
    godep save

The .travis.yml file has been updated to use godep for building
binaries and running tests. The prepare.sh script has been updated
to use godep to build the cadvisor binary.
Merge pull request #135 from kelseyhightower/godep

manage dependencies with godep