PkgEval: assumes 'master' branch name

[maleadt]

Nanosoldier.jl/src/jobs/PkgEvalJob.jl

Lines 160 to 162 in 26ce3fb

	merge_base = GitHub.compare(submission.repo,
	"master", "refs/pull/$(submission.prnumber)/head";
	auth=submission.config.auth).merge_base_commit

Results in: JuliaLang/PrecompileTools.jl#13 (comment)