comments: Allow whitespace after the shebang marker

Basically, any character is now allowed after the shebang marker.

Closes #428.

Whitespace after the #! marker on shebang lines is authorized and
optional, as explained on Wikipedia's entry for shebang line as can be
seen from the extracts below :

> White space after #! is optional

and

> It has been claimed[20] that some old versions of Unix expect the
> normal shebang to be followed by a space and a slash (#! /), but this
> appears to be untrue;[21] rather, blanks after the shebang have
> traditionally been allowed, and sometimes documented with a space

yamllint/rules/comments.py

@@ -74,8 +74,6 @@ Use this rule to control the position and formatting of comments.
 """
 
 
-import re
-
 from yamllint.linter import LintProblem
 
 
@@ -105,7 +103,7 @@ def check(conf, comment):
             if (conf['ignore-shebangs'] and
                     comment.line_no == 1 and
                     comment.column_no == 1 and
-                    re.match(r'^!\S', comment.buffer[text_start:])):
+                    comment.buffer[text_start] == '!'):
                 return
             # We can test for both \r and \r\n just by checking first char
             # \r itself is a valid newline on some older OS.