Pass 'unicode' to re:run/3

Otherwise the application crash if the input string contains multibyte
characters. Here's the backtrace:
  2> yamerl_constr:string("いろは").
  ** exception error: bad argument
       in function  re:run/3
          called as re:run([12356,12429,12399],
                           "^(\\.[0-9]+|[0-9]+(\\.[0-9]*)?)([eE][-+]?[0-9]+)?$",
                           [{capture,none}])
       in call from yamerl_node_float:string_to_float2/1 (src/yamerl_node_float.erl, line 145)
       in call from yamerl_node_float:construct_token/3 (src/yamerl_node_float.erl, line 64)
       in call from yamerl_node_float:try_construct_token/3 (src/yamerl_node_float.erl, line 54)
       in call from yamerl_constr:try_construct/3 (src/yamerl_constr.erl, line 371)
       in call from yamerl_constr:construct/2 (src/yamerl_constr.erl, line 315)
       in call from yamerl_constr:string/2 (src/yamerl_constr.erl, line 203)

PR:		#5
Reported by:	Eric Sagnes (ericsagnes on GitHub)