pkgdown: remove unnecessary Rbuildignore rule

.Rbuildignore

@@ -3,4 +3,4 @@
 ^README\.Rmd$
 ^_pkgdown\.yml$
 ^public$
-^pkgdown$
+^\.gitlab-ci\.yml

DESCRIPTION

 Package: spreadrate
 Type: Package
 Title: Estimate the spread-rate of epidemics
-Version: 0.1.6
-Date: 2019-07-17
+Version: 0.1.7
+Date: 2019-11-04
 Authors@R: person("Facundo", "Muñoz", email = "facundo.munoz@cirad.fr",
         role = c("aut", "cre"), comment = c(ORCID =
         "0000-0002-5061-4241"))