Upgrade doc/make.jl to introspect branch/tag names from git info

[staticfloat]

Note that this requires that the julia you're using to deploy the docs matches the current julia checkout.

[mortenpi]

Also needs TRAVIS_PULL_REQUEST = "false" when building on branches. It should be fine to just set it iff on CI -- the tag and branch checks should make sure that we only deploy on non-PR builds.

[staticfloat]

What happens if TRAVIS_PULL_REQUEST is not set? I was planning on just only running make -C doc deploy when on master or release-*.

[mortenpi]

It won't deploy. If we already check for PRs, then TRAVIS_PULL_REQUEST could just always be set to false.

The only minor this is that deploydocs will also try to deploy when running make deploy locally then (because all the env.vars are now set; well, it will fail due to a missing DOCUMENTER_KEY most likely). But that's probably OK.. the average user is not expected to run make -C doc deploy anyway I think.

[mortenpi]

Suggested change

	"TRAVIS_BRANCH" => Base.GIT_VERSION_INFO.branch,
	"TRAVIS_BRANCH" => Base.GIT_VERSION_INFO.branch,
	"TRAVIS_PULL_REQUEST" => "false",

[staticfloat]

The only minor this is that deploydocs will also try to deploy when running make deploy locally then

Ah, I see. In that case, let's set TRAVIS_PULL_REQUEST to false within the buildbot configuration.

[staticfloat]

I'm going to merge this, then we'll see if the build on master goes through properly. :D