Use travis

[ning-y]

Once #6 is implemented, travis should run make all and compare the diffs to make sure docs/*.png and pdf/*.pdf have been updated.

---

Looks like two compiled pdfs can't be the same, because the cdate and mdate are embedded in the file:

ning:~/temp $ diff test.pdf  test1.pdf --text
57,58c57,58
< /CreationDate (D:20180922113906+08'00')
< /ModDate (D:20180922113906+08'00')
---
> /CreationDate (D:20180922113859+08'00')
> /ModDate (D:20180922113859+08'00')
84c84
< /ID [<09E48BCF9B8804B9F2D051A74DE46D23> <09E48BCF9B8804B9F2D051A74DE46D23>]
---
> /ID [<F1EE7BE89A8172899D208856FCCB9E28> <F1EE7BE89A8172899D208856FCCB9E28>]

I can probably write a script to do this diff --text and parse its output to make sure nothing but cdate, mdate, and id has changed.