fix <c-y> causing strange popup menu and solve <tab> conflict with ultisnips

Use <c-y> to accept the current candidate will cause a second popup menu. Because after ycm insterts the candidate, cursor position has changed and ycm will find a new candidate again. I delete the candidate exactly same as the text to match. Thus, no second popup menu will occur.

By this enhancement, I can use vim script to use <tab> for both cycling through ycm candidates and ultisnips placeholders. Also I can use <enter> for both accepting ycm candidate and expand ultisnips. I add the solution to README.md.