Install stringi with install.packages() on macOS. [skip appveyor]

Both stringi and knitr have source versions that are more recent than
their corresponding binary versions. When the remotes package tries to
install them from source, it attempts to install knitr first. Since
knitr depends on stringi (via stringr), the installation fails.

r-lib/remotes#296

install.packages() installs the source packages in the correct order.

.travis.yml

@@ -26,12 +26,14 @@ matrix:
       warnings_are_errors: false
     - os: osx
       r: oldrel
+      r_packages: stringi
       addons:
         homebrew:
           packages:
             - libgit2
     - os: osx
       r: release
+      r_packages: stringi
 
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