Use POSIX-Compliant Command-Exists Test

Suggest the `command` command instead of `which` to determine if the `lsb_release` command exists.
`which` is a non-standard command and this script and its documentation should work across as many POSIX like systems as possible.
`command` is more portable than `which`.

To be consistent with the existing suggestion, the updated suggestion will only act if `lsb_release` exists and is a simple command, not an alias or shell function.

References:
http://stackoverflow.com/a/677212/418413
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

README.rst

@@ -392,7 +392,7 @@ Please run the following commands and report their output when creating a ticket
 .. code:: console
 
   sudo find /etc/ -name '*-release' -print -exec cat {} \;
-  which lsb_release && lsb_release -a
+  command lsb_release -a
 
 
 Testing in Vagrant