shell: Replace ‘which’ utility with ‘command -v’ shell builtin.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

Vagrantfile

@@ -2,13 +2,8 @@
 
 VAGRANTFILE_API_VERSION = "2"
 
-def command?(name)
-  `which #{name} > /dev/null 2>&1`
-  $?.success?
-end
-
 if Vagrant::VERSION == "1.8.7"
-  path = `which curl`
+  path = `command -v curl`
   if path.include?("/opt/vagrant/embedded/bin/curl")
     puts "In Vagrant 1.8.7, curl is broken. Please use Vagrant 2.0.2 " \
          "or run 'sudo rm -f /opt/vagrant/embedded/bin/curl' to fix the " \

tools/ci/Dockerfile.template

@@ -59,7 +59,7 @@ RUN set -e \
   && tar -xz -C /tmp -f /tmp/docker.tgz \
   && mv /tmp/docker/* /usr/bin \
   && rm -rf /tmp/docker /tmp/docker.tgz \
-  && which docker \
+  && command -v docker \
   && (docker version 2>/dev/null || true)
 
 # ...docker-compose...