Fixes bug in MASExampleUpdateView

[ShannonChenCHN]

This is a bug found in iOS Example Project. When I tapped the growingButton in MASExampleUpdateView, the button didn't grow. Probably we need to change the width constraint priority of growingButton from low to medium. I have tried this fix on my project, and it worked fine.

When I printed all constraints of growingButton, and I got this:

[self.growingButton updateConstraints:^(MASConstraintMaker *make) {
        make.center.equalTo(self);
        make.width.equalTo(@(self.buttonSize.width)).priorityLow();
        make.height.equalTo(@(self.buttonSize.height)).priorityLow();
        make.width.lessThanOrEqualTo(self);
        make.height.lessThanOrEqualTo(self);
    }];

    for (NSLayoutConstraint *constraint in self.growingButton.constraints) {
        NSLog(@"---- priority : %@, %@\n\n", constraint, @(constraint.priority));
    }

    /**
     priority : <MASLayoutConstraint:0x6000000b8ae0 UIButton:0x7f81ea51a290.width == 130 ^low>, 250

     priority : <MASLayoutConstraint:0x6000000b8b40 UIButton:0x7f81ea51a290.height == 130 ^low>, 250

     priority : <NSContentSizeLayoutConstraint:0x6040000b4fa0 UIButton:0x7f81ea51a290.width == 66>, 1000

     priority : <NSContentSizeLayoutConstraint:0x6040000b5000 UIButton:0x7f81ea51a290.height == 30>, 1000
     */

So, It looks like make.width.lessThanOrEqualTo(self); makes a constraint with a default priority( NSLayoutPriorityRequired).