Bump API version number

... due to the incompatible change on the Session.login_with_password signature etc.

Xapi_globs.xencenter_min_verstring is kept unchanged and should probably wait until XenCenter changes get in.

Signed-off-by: Zheng Li <zheng.li@eu.citrix.com>
Merge pull request #1154 from zli/xenserver/api-version-number

Bump API version number