Try to support older R versions

.travis.yml

@@ -2,7 +2,9 @@
 
 # Setting up R deps
 language: r
-r: 3.1
+r:
+- 3.1
+- 3.2
 r_packages: covr
 cache: packages
 
@@ -14,6 +16,7 @@ before_install:
   - if [ $TRAVIS_OS_NAME = linux ]; then sudo apt-get -qq update; fi
   - if [ $TRAVIS_OS_NAME = linux ]; then sudo apt-get install -y xclip; fi
   - if [ $TRAVIS_OS_NAME = osx ]; then brew install xclip; fi
+  - if [ $TRAVIS_OS_NAME = osx ]; then brew install r@3.1.3; fi
 
 # postrun
 after_success:

R/freq.R

@@ -161,7 +161,7 @@ freq <- function(x,
   }
 
   if (mult.columns > 1) {
-    NAs <- x[is.na(x) | x == trimws(strrep('NA ', mult.columns))]
+    NAs <- x[is.na(x) | x == trimws(AMR:::strrep('NA ', mult.columns))]
   } else {
     NAs <- x[is.na(x)]
   }