travis: workaround go-get-insecure for rsc.io/pdf

[sbinet]

while the go get rsc.io/pdf failure is fixed upstream
(rsc/pdf#4), check out manually the repository from github and put it
in the correct place under $GOPATH.

[eaburns]

LGTM.

[eaburns]

Actually, can you pass -insecure to go get instead?
It works for me locally:

$ go get rsc.io/pdf
package rsc.io/pdf: unrecognized import path "rsc.io/pdf" (https fetch: Get https://rsc.io/pdf?go-get=1: x509: certificate signed by unknown authority)
$ go get -insecure rsc.io/pdf
$ echo $?
0

[sbinet]

you could, but go get -insecure was introduced some time after 1.4.3.
I am not sure if #define are possible in .travis.yml files... :)

[eaburns]

OK then this is fine.