treewide: add lexical binding directive to *.el files

Fixes a bunch of warnings like:
    …/.yas-setup.el:1:1: Warning: file has no ‘lexical-binding’ directive on its first line

snippets/bibtex-mode/.yas-setup.el

@@ -1,3 +1,4 @@
+;;; -*- lexical-binding: t -*-
 (require 'yasnippet)
 (require 'yasnippet-snippets)
 

snippets/c++-mode/.yas-setup.el

@@ -1,3 +1,4 @@
+;;; -*- lexical-binding: t -*-
 (require 'yasnippet)
 
 (defun yas-c++-class-name (str)

snippets/julia-mode/.yas-setup.el

@@ -1,3 +1,4 @@
+;;; -*- lexical-binding: t -*-
 (require 'yasnippet)
 
 (defun yas-julia-iteration-keyword-choice ()

snippets/php-mode/.yas-setup.el

@@ -1,4 +1,4 @@
-;;; .yas-setup.el --- Yasnippet helper functions for PHP snippets
+;;; .yas-setup.el --- Yasnippet helper functions for PHP snippets -*- lexical-binding: t -*-
 
 ;;; Commentary:
 

snippets/prog-mode/.yas-setup.el

@@ -1,3 +1,4 @@
+;;; -*- lexical-binding: t -*-
 (require 'yasnippet)
 
 ;; whitespace removing functions from Magnar Sveen ;;;;;;;;;;;;;;;;;;;;;;;;;;;;

snippets/python-mode/.yas-setup.el

@@ -1,3 +1,4 @@
+;;; -*- lexical-binding: t -*-
 (require 'yasnippet)
 (defvar yas-text)
 

snippets/rjsx-mode/.yas-setup.el

@@ -1,4 +1,4 @@
-;;; .yas-setup.el --- Yasnippet helper functions for JSX snippets
+;;; .yas-setup.el --- Yasnippet helper functions for JSX snippets -*- lexical-binding: t -*-
 
 ;;; Commentary:
 

yasnippet-snippets.el

@@ -1,4 +1,4 @@
-;;; yasnippet-snippets.el --- Collection of yasnippet snippets
+;;; yasnippet-snippets.el --- Collection of yasnippet snippets -*- lexical-binding: t -*-
 
 ;; Copyright (C) 2017 Andrea Crotti