Use GET instead of POST for Jenkins build request.

I don't know why this isn't a problem for anyone else. When leeroy makes build request to Jenkins using POST, it fails with a status 411 from Jenkins. The reason for the 411 is the POST has no content-length. The error in leeroy's log looks like the following.
```
Requesting build from Jenkins: http://jenkins.example.com/job/leeroy-testing/buildWithParameters?GIT_BASE_REPO=cluther/leeroy-testing&GIT_HEAD_REPO=cluther/leeroy-testing&GIT_SHA1=a00f4cae71d5b7399bd8c43fd04bad2b2dc32dc4&GITHUB_URL=https://github.com/cluther/leeroy-testing/pull/1
Jenkins responded with status code 411
```

Changing the POST to a GET fixes it. Considering there's no data in the post, it doesn't seem like this would cause problems.