Try to fix Travis.

I'm guessing that trying to write to benchmark.txt inside travis is likely creating the error due to permissions. Any file changes inside Travis will be lost anyway, so there's no point in doing that.
Instead, I just opted to execute the test and benchmarks and show their output in Travis.
Also make sure the Go code is gofmted.
Fix Travis.

Okay, it turns out the issue was a missing dependency.
Using make obscured the actual error, which is one reason to avoid using it.
Merge pull request #4 from shurcooL/patch-1

Try to fix Travis.