Replace regexp for matching regexeps with atom's version.

[graceful-potato]

Potential solution for #117

[graceful-potato]

looks like /\\/ is not highlighted by this rule. And I think that this can be fixed with
(?:\\\\|\\/|[^/])*+

[graceful-potato]

Also this regexp has some disadvantages with multi-line expression.

If you want to write division and put divisor on the new line then you have to use a whitespace after /. Whithout whitespace this expression will be highlighted as regexp.

# without whitespace after `/`
# `/` will be highlighted as start of regexp
4 /
2

# with whitespace after `/`
# expression will be highlighted as division
4 / 
2

If you want to write multi-line regexp then you have to make sure that there isn't a single character after /

# with whitespace after `/`
# expression will not be highlighted as multi-line regex
regexp = /  
foo
bar
/

# with any other character after `/`
# expression will not be highlighted as multi-line regex
regexp = /f
oo
bar
/

# without any character after `/`
# expression will be highlighted as multi-line regex
regexp = /
foo
bar
/

Github highlighting has exactly the same issues.
UPDATE
Same issues only in preview mode

[graceful-potato]

Well, according to issue atom/language-ruby#82 this regexp will bring in new highlighting problems.