VersionInfo constructor should use keyword arguments for prerelease and build with default set as None

[scls19fr]

https://github.com/k-bx/python-semver/blob/d964b1668cb9ae0d941b9fdbec9f3b42121805c3/semver.py#L86

should be

 def __init__(self, major, minor, patch, prerelease=None, build=None): 

Code showing problem:

In [1]: from semver import VersionInfo

In [2]: VersionInfo(1, 2, 3)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-f524b302b2ba> in <module>()
----> 1 VersionInfo(1,2,3)

TypeError: __init__() missing 2 required positional arguments: 'prerelease' and 'build'

In [3]: VersionInfo(1, 2, 3, None, None)
Out[3]: VersionInfo(major=1, minor=2, patch=3, prerelease=None, build=None)