fix color hook for newer knitr versions

Newer knitr injects \usepackage[]{xcolor} instead of \usepackage[]{color},
so the old hook never matched and dvipsnames colors (e.g. Blue) were undefined.

paper_template/text.Rnw

@@ -12,8 +12,11 @@
 
 <<init, echo=FALSE>>=
 knit_hooks$set(document = function(x) {
-  sub('\\usepackage[]{color}',
+  x <- sub('\\usepackage[]{color}',
            '\\usepackage[usenames,dvipsnames]{color}', x, fixed = TRUE)
+  x <- sub('\\usepackage[]{xcolor}',
+           '\\usepackage[usenames,dvipsnames]{xcolor}', x, fixed = TRUE)
+  x
 })
 opts_chunk$set(fig.path="figures/knitr-")