(bugfix) Test whether a method is supported by mediawiki version

The given assert in some generators does not work because
Site.versionnumber is a function, not an attribute holding the
value.

The assert function is replaced by the decorator "need_version"
to require a certain mediawiki version number.

Change-Id: I40460e83189954a7b2d0b37e5198084993d1a5a4
Merge "(bugfix) Test whether a method is supported by mediawiki version"