major/minor bumps should bump to the next major/minor version. Ref #94.

As per https://github.com/npm/node-semver/issues/94 this pull-request changes the behaviour of `inc( v, 'major')` and `inc( v, 'minor')` when on a pre-major or pre-minor release respectively. 

In particular when on `X.0.0-something`, `major` bump will take you to the next major version, `X.0.0`.

Similarly when on `X.Y.0-something`, `minor` bump will take you to the next minor version, `X.Y.0`.

Unit tests added. All unit tests passed.