Only !merge local vagrant params when present

If vagrantconfig_local.yaml is empty `YAML.load()` will eval to false and upon
running `vagrant up` you will get the error: "ERROR loader: Vagrantfile load
error: no implicit conversion of false into Hash." This patch checks to make
sure some YAML was parsed before merging it.
Merge branch 'master' of github.com:mozilla/dxr into bug-985260
Refactored single line and range detection into a tidy little generateSelectedArrays() function