Removed erroneous section numbers from Volume 4.
Added a separate volume for labs no longer included in Volumes 1 - 4.
Added extra labs to the Travis build.
Merge pull request #670 from insertinterestingnamehere/sections

Removed erroneous section numbers from Volume 4.
Added B-trees lab to extra labs volume.
Fixed typo in travis build file.
Merge pull request #673 from insertinterestingnamehere/extra_labs

Orphaned labs
Changed travis build so that a failed assertion of the existence of a
Volume's pdf file results in a test failure.
Add assertion of existence of pdf for extra labs to Travis build.
Merge pull request #676 from insertinterestingnamehere/travis

Travis