Use -t with go get for Go repos

I noticed that my repo failed Travis here: https://travis-ci.org/natefinch/lumberjack/builds/27889216

The problem is that I added packages to a test file that don't get referenced by the non-test files.  By default, go get doesn't download those packages, unless you pass it -t.  So, for CI, -t should always be specified.  

Note that after I changed the install step to use -t, the build passes, despite no other changes to the repo:
https://travis-ci.org/natefinch/lumberjack/builds/27890182

For any existing Go repos, adding -t won't hurt anything if they're already passing.

I can make a separate PR for the docs if this one is approved.