Define xref methods only for emacs versions >= 25 (possibly fixes xref support)

[vibhavp]

No description provided.

[Wilfred]

I've pushed an xref_support branch with the conflicts merged. There's some code duplication though, e.g. racer--xref-make-definitions is manually calling racer rather than using racer--call.

If you'd like to take a look, that would be a great help. I do want us to merge this, so I will get around to it, but it's not currently top of my list.

[vibhavp]

LGTM

[Hi-Angel]

@Wilfred so, should xref work at the moment? For me calling xref-find-definitions for the same symbol where racer-find-definition worked results in a prompt for a TAGS file.