[ISSUE-37] [ISSUE-82] Encode trace message before update build

First off I am not a Ruby developer so please bare with me if this might not be the solution to go.
This PR fixes a problem where the build trace being sent to the coordinator during update build breaks the runner resulting in an infinite running build (see issues https://github.com/gitlabhq/gitlab-ci-runner/issues/37 and https://github.com/gitlabhq/gitlab-ci-runner/issues/82.
The problem is if the build trace contains invalid UTF-8 byte sequences, the `PUT` request fails during update build and leaves the runner in an inconsistent state where it cannot break out again.
It's just outputting `Submitting build 379 to coordinator...failed` over and over again.
The result is that the runner has to be killed and restarted.
I adopted a fix suggested [here](http://robots.thoughtbot.com/fight-back-utf-8-invalid-byte-sequences) to come around this issue.