Revert "Use ''' instead of ''', which showed in template output"

As reported by Chad Weider (Thanks!), the apostrophe should not be escaped as
"'" in HTML documents, but using a numeric entity "'" or "&#x27".

Unlike """, "'" has not beed defined in the W3C Recommendation
for HTML 4 [1].

References:

[1] HTML 4.01 Specification
§24.4.1 The list of characters
http://www.w3.org/TR/html4/sgml/entities.html#h-24.3.1

[2] Why shouldn't `'` be used to escape single quotes?
http://stackoverflow.com/questions/2083754
  /why-shouldnt-apos-be-used-to-escape-single-quotes

[3] OWASP - The Open Web Application Security Project
RULE #1 - HTML Escape Before Inserting Untrusted Data into HTML Element Content
"' is not recommended"
https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet#RULE_.231_-_HTML_Escape_Before_Inserting_Untrusted_Data_into_HTML_Element_Content

See details of the discussion on Github issue tracker:
https://github.com/janl/mustache.js/pull/166

This reverts commit e20cee5f3e.

Conflicts:

	mustache.js

mustache.js

@@ -40,7 +40,7 @@ var Mustache = function () {
     "<": "&lt;",
     ">": "&gt;",
     '"': '&quot;',
-    "'": '&apos;'
+    "'": '&#39;'
   };
 
   function escapeHTML(string) {