Fix: unput leaves yytext one character short

This patch landed in jison about a year ago (https://github.com/zaach/jison/pull/135/files) but never made its way to jison-lex. Hence newer versions of jison (that use jison-lex) are broken again.
Merge pull request #8 from fgnass/patch-1

Fix: unput leaves yytext one character short