Rhub checks for Ubuntu Linux 16.04 LTS failed

[villegar]

* checking tests ...
  Running ‘testthat.R’ [15s/54s]
 ERROR
Running the tests in ‘tests/testthat.R’ failed.
Last 13 lines of output:
  
  ══ testthat results  ═══════════════════════════════════════════════════════════
  ERROR (test-utils-plot.R:89:3): PCA works
  Warning (test-utils-qtl.R:94:3): effect plots work
  ERROR (test-utils-qtl.R:96:3): effect plots work
  Warning (test-utils-qtl.R:153:3): read cross file works
  ERROR (test-utils-qtl.R:154:3): read cross file works
  FAILURE (test-utils.R:5:3): check data types works
  FAILURE (test-utils.R:6:3): check data types works
  FAILURE (test-utils.R:7:3): check data types works
  FAILURE (test-utils.R:11:3): check data types works
  
  [ FAIL 7 | WARN 2 | SKIP 0 | PASS 264 ]
  Error: Test failures
  Execution halted