Bumps paas-admin version to 0.51.0
Bumps paas-admin version to 0.51.0

The bug fix issused for #162443556 incremented the paas-admin version to 0.51.0. This commit bumps the version in paas-cf to match.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf