[BUGFIX] Improve resolution of core migration ``version`` argument
By specifying the ``version`` flag on the ``./flow core:migrate``
command a specific core migration can be targeted.
This change improves the regular expression that extracts the numeric
identifier from that flag:
Previously when specifying the whole version string as in
``TYPO3.Fluid-20150214130800`` the resulting version number would
include the "3" of the package key.
Releases: master, 3.0