Fix version specification in apt pkg.install

Changing the install command to use shell=False in the call to
subprocess.Popen caused version specification to stop working due to the
"pkgname=version" string being quoted to prevent shell evaluation of
possible "<" or ">" characters. However, with shell=False this is no
longer a concern, so this no longer needs to be quoted.

Also switched to using raw strings where applicable, instead of manually quoting them.