Fix ordering

[michamos]

• Figures are now listed in the order in which they appear in the document. This only works reliably in the (probably most common) case where there is only one LaTeX source file, as no attempt is done to understand the ordering in which subdocuments are included in the main document (with \input, \include, etc.). If there are multiple source files, the ordering of them is random, but figures will be ordered correctly within each source files, which is already an improvement over the current random order.

• INSPIR-2865

[michamos]

Build is green, but travis reporting failed somehow: https://travis-ci.org/inspirehep/plotextractor/builds/602322277