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

[deeky666]

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 to come around this issue.

[jvanbaarsen]

@randx Looks good to merge