Allow specifying version in depends_on :python

Note, in the explict form:

    PythonInstalled.new('2.7') => :recommended

the tag :recommended is ignored (not a limitation
of PythonInstalled itself). One solution was to write

    PythonInstalled.new('2.7', [:recommended])

but that is not as beautiful as we like it.

Therefore, now it is possible to:

    depends_on :python => ['2.7', :recommended]

Only the first tag is attempted to be parsed as
a version specifyer "x" or "x.y" or "x.y.z"...