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

[Liangzhi233]

Use CONTROL-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 key for both cycling through ycm candidates and ultisnips placeholders. Also I can use ENTER key for both accepting ycm candidate and expand ultisnips. I add the solution to README.md.

[vheon]

Travis is not happy, apparently some tests are not passing. What I didn't understand is what you are trying to solve. Can you make a simple reproducible test case?

[Liangzhi233]

Look at this situation, when I typed "al" and select the right candidates. If I use CONTROL-Y to accept the candidate and close the popup menu, I get another popup menu, like following picture

This strange popup menu can be closed by hand by one more CONTROL-Y, but can never be closed by vim script. Thus, I cannot use TAB to cycle through placeholders in ultisnips.

My solution is to make ycm not display the candidate exact same as the text I have typed.

[Valloric]

The benefit doesn't seem worth the extra complexity, sorry. Thank you for sending a pull request though!