Fixed SilverStripe detection on non-numeric versions

The old version detection broke the Travis builds for forks, since the 'version'
in this case is the branch name, which can't be compared
with `version_compare()`. This lead to the core folder always
being renamed to `sapphire/` (instead of leaving it with
the newer naming of `framework/`). Which in turn caused test
execution failure because the bootstrap path wasn't found.

The patch doesn't guard against branch names which are based off 2.x
(with the old `sapphire/` folder naming, since composer doesn't have this
branch point information, but rather adapts to the most common usage.

/cc @sminnee