Fix flakey tests where scan fails to find & label Test PR

[rtyley]

Fixes #145 - the issue was caused by the slightly variable results returned by the GitHub API.

In all the tests in FunctionalSpec, we create several test fixtures with the GitHub API, each created to be used once and thrown away:

• A brand-new GitHub repository, populated with whatever Git history we require
• A brand new PR in the repository, which we also merge

Before continuing on to the next step, which would be having Prout scan the repo for recently-merged PRs, we perform several checks to ensure that the GitHub API confirms the status of the merged PR:

prout/test/lib/Helpers.scala

Lines 180 to 195 in a710e34

	eventually {
	whenReady(repo.pullRequests.get(pr.number)) {
	_.mergeable.value mustBe true
	}
	}
	eventually {
	whenReady(pr.merge(MergePullRequest())) { _.merged must be(true) }
	}
	val mergedPR = eventually {
	whenReady(repo.pullRequests.get(pr.number)) {
	pr => pr.merged_by mustBe defined ;
	logger.info("Created and merged PR: "+pr.html_url)
	pr }
	}

However, looking a the differing logs produced by the failure & success cases in #145, it became apparent that even though we've checked the GitHub API.