Same issue like #244 . I tried to install some package from private repository.
It requires angular-ui/angular-google-maps.
But in bower.json file of angular-google-maps package we can see:

"dependencies": {
    ...
    "lodash": "3.X - 4.X",
    ...
}

and that is why I got the error:
Could not parse version constraint >=3.x: Invalid version string "3.x".

So I ran Fxp\Composer\AssetPlugin\Tests\Converter\SemverConverterTest and provided one more example in data-provider method.
I expected that '3.x - 4.x' would be converted to '>=3.0,<5.0', but it didn't:

Failed asserting that two strings are equal.
Expected :'>=3.0,<5.0'
Actual :'>=3.x,<5.0'

Why does SemverConverter replace only second part of the range "3.x - 4.x" ?