Update README to reflect the need for 64-bit build env

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #483 from simonjbeaumont/64-bit-readme

Update README to reflect the need for 64-bit build env
specdep: Switch back to using argparse

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #489 from euanh/specdep-ignore

specdep: Switch back to using argparse
specdep: Fix default value for --dist argument

Signed-off-by: Euan Harris <euan.harris@citrix.com>