Bug: Constraints::Check(...) function mistakenly returns false, expected true

Hey,

the following example shows an unexpected behavior of the constraints' check function:

```golang
v1 := MustVersion("1.7.3")
v2 := MustVersion("1.7.4~pre.0")
v3 := MustVersion("1.7.4")
if !v1.LessThan(v2) {
panic("v1 not less than v2")
}
if !v2.LessThan(v3) {
panic("v2 not less than v3")
}
if !v1.LessThan(v3) {
panic("v1 not less than v3")
}
c := MustConstraint(">= 1.7.3")
if !c.Check(v1) {
panic("constraint failed for v1")
}
if !c.Check(v3) {
panic("constraint failed for v3")
}
if !c.Check(v2) {
panic("constraint failed for v2")
}
```

My output is `constraint failed for v2`. The above code shows, that `1.7.3` is less than `1.7.4~pre.0` but `1.7.4~pre.0` does not match constraint `>= 1.7.3`.

Tested commit: *v1.1.0* `d40cf49b3a77bba84a7afdbd7f1dc295d114efb1`

Thanks,
Marcel Gebhardt