Fix RETableViewBoolCell's and RETableViewSegmentedCell's layout on iOS 8

On iOS 8 bool and segmented cells had broken layout when they were redisplayed (when `cellWillAppear` was called more than one time):
![screenshot 2014-06-21 18 41 44](https://cloud.githubusercontent.com/assets/197767/3349428/f3e80636-f962-11e3-846a-7253faf14221.png)

The problem was with removal of all `contentView's` constraints in `cellWillAppear`. In iOS 8 UIKit adds its own constraints and removing them causes glitches. I tested the fix on iOS 6.1-8.0b2.

 Here's the comparison of the constraints between iOS versions with breakpoint set on the first line of `cellWillAppear` in `RETableViewSegmentedCell`:

**iOS 7**
```
(lldb) po self.contentView.constraints
<__NSArrayM 0x8c71fb0>(
<NSLayoutConstraint:0x8c8b3b0 UISegmentedControl:0x8f5f1d0.centerY == UITableViewCellContentView:0x8cb17f0.centerY>,
<NSLayoutConstraint:0x8c8b530 H:|-(15)-[UISegmentedControl:0x8f5f1d0]   (Names: '|':UITableViewCellContentView:0x8cb17f0 )>,
<NSLayoutConstraint:0x8c8b580 H:[UISegmentedControl:0x8f5f1d0]-(15)-|   (Names: '|':UITableViewCellContentView:0x8cb17f0 )>
)
```

**iOS 8**

```
(lldb) po self.contentView.constraints
<__NSArrayM 0xaf279b0>(
<NSLayoutConstraint:0xaf4f4e0 UISegmentedControl:0xc313110.centerY == UITableViewCellContentView:0xc303560.centerY>,
<NSLayoutConstraint:0xaf24920 H:|-(15)-[UISegmentedControl:0xc313110]   (Names: '|':UITableViewCellContentView:0xc303560 )>,
<NSLayoutConstraint:0xaf22800 H:[UISegmentedControl:0xc313110]-(15)-|   (Names: '|':UITableViewCellContentView:0xc303560 )>,
<NSLayoutConstraint:0xc30cb10 'UIView-Encapsulated-Layout-Width' H:[UITableViewCellContentView:0xc303560(320)]>,
<NSLayoutConstraint:0xc32a7a0 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0xc303560(43.5)]>,
<NSAutoresizingMaskLayoutConstraint:0xc30b0c0 h=--& v=--& 'UIView-Encapsulated-Layout-Left' H:|-(0)-[UITableViewCellContentView:0xc303560]   (Names: '|':RETableViewSegmentedCell:0xc3169c0'RETableViewManager_RESegm...' )>,
<NSAutoresizingMaskLayoutConstraint:0xc34c1b0 h=--& v=--& 'UIView-Encapsulated-Layout-Top' V:|-(0)-[UITableViewCellContentView:0xc303560]   (Names: '|':RETableViewSegmentedCell:0xc3169c0'RETableViewManager_RESegm...' )>
)
```