title: Error in unloadNamespace(package) : namespace 'MASS' is imported by 'vegan' so cannot be unloaded
body: <pre><code>checkpoint("2019-12-11",R.version = "3.5.3",scanForPackages = FALSE,verbose = TRUE,use.knitr=FALSE, auto.install.knitr = FALSE, scan.rnw.with.knitr = TRUE,forceInstall = FALSE, forceProject = FALSE, use.lock = TRUE)
  
  r$&gt; packageVersion('checkpoint')
  [1] '0.4.9'
  
  r$&gt; library(knitr)
  
  r$&gt; library(rmarkdown)
  
  r$&gt; library(MASS)
  Error in value[[3L]](cond) : 
    Package 'MASS' version 7.3.51.3 cannot be unloaded:
   Error in unloadNamespace(package) : namespace 'MASS' is imported by 'vegan' so cannot be unloaded
  </code></pre>
  
  <p>When I face this error,I <code>remove.packages('MASS')</code> and reinstall,but cannot solve this problem.<br>
  How to solve this problem?</p>