IMPALA-3767: bootstrap_virtualenv fails to find cython distribution
IMPALA-3767: bootstrap_virtualenv fails to find cython distribution

This patch avoids trying to download a Cython binary 0.23.4, which pip
has trouble finding.

Change-Id: Ic6733ccb71bcf99196075faa2fb6cf2a1d6276ce
Reviewed-on: http://gerrit.cloudera.org:8080/3427
Reviewed-by: Jim Apple <9e3620e7251463d214b5f536438d2c9ea899162d@cloudera.com>
Tested-by: Internal Jenkins

cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala