Replace "which" with "command -v"

Since the latter is at least semi-POSIX compliant:

http://pubs.opengroup.org/onlinepubs/009696899/utilities/command.html

Avoids the need for certbot#1486

bootstrap/_deb_common.sh

@@ -44,7 +44,7 @@ apt-get install -y --no-install-recommends \
   libffi-dev \
   ca-certificates \
 
-if ! which virtualenv > /dev/null ; then
+if ! command -v virtualenv > /dev/null ; then
   echo Failed to install a working \"virtualenv\" command, exiting
   exit 1
 fi

letsencrypt-auto

@@ -83,11 +83,11 @@ ExperimentalBootstrap() {
 }
 
 DeterminePythonVersion() {
-  if which python2 > /dev/null ; then
+  if command -v python2 > /dev/null ; then
     export LE_PYTHON=${LE_PYTHON:-python2}
-  elif which python2.7 > /dev/null ; then
+  elif command -v python2.7 > /dev/null ; then
     export LE_PYTHON=${LE_PYTHON:-python2.7}
-  elif which python > /dev/null ; then
+  elif command -v python > /dev/null ; then
     export LE_PYTHON=${LE_PYTHON:-python}
   else
     echo "Cannot find any Pythons... please install one!"